7 ms·

Understanding ZFS Scrubs and Data Integrity

93 points by zdw 10mo ago

itchingsphynx 9mo ago
>Most systems that include ZFS schedule scrubs once per month. This frequency is appropriate for many environments, but high churn systems may require more frequent scrubs. Is there a more specific 'rule of thumb' for scrub frequency? What variables should one consider?
- nubinetwork 9mo ago
  Total pool size and speed. Less data scrubs faster, as do faster disks or disk topology (a 3 way stripe of nvme will scrub faster than a single sata ssd) For what its worth, I scrub daily mostly because I can. It's completely overkill, but if it only takes half an hour, then it can run in the middle of the night while I'm sleeping.
- ssl-3 9mo ago
  The cost of a scrub is just a flurry of disk reads and a reduction in performance during a scrub. If this cost is affordable on a daily basis, then do a scrub daily. If it's only affordable less often, then do it less often. (Whatever the case: It's not like a scrub causes any harm to the hardware or the data. It can run as frequently as you elect to tolerate.)
  - agapon 9mo ago
    With HDDs, it's also mechanical wear and increased chance of a failure. SSDs are not fully immune to increased load either.
    - ssl-3 9mo ago
      Is there any evidence that suggests that reading from a hard drive (instead of it just spinning idle) increases physical wear in any meaningful way? Likewise, is there any evidence of this for solid-state storage?
      - rcxdude 9mo ago
        Yes. Hard drives have published "Annualized Workload Rate" ratings, which are in TB/year, and the manufacturers state there is no difference between reads and writes for the purpose of this rating. (https://www.toshiba-storage.com/trends-technology/mttf-what-hard-drive-reliability-really-means/ https://www.toshiba-storage.com/trends-technology/mttf-what-...) For SSDs, writes matter a lot more. Reads may increase the temperature of the drive, so they'll have some effect, but I don't think I've seen a read endurance rating for an SSD.
      - digiown 9mo ago
        Reading from it requires the read head to move, as opposed to spinning idle where the heads are parked on the side. Moving parts generally wear out over time.
- toast0 9mo ago
  Once a month seems like a reasonable rule of thumb. But you're balancing the cost of the scrub vs the benefit of learning about a problem as soon as possible. A scrub does a lot of I/O and a fair amount of computing. The scrub load competes with your application load and depending on the size of your disk(s) and their read bandwidth, it may take quite some time to do the scrub. There's even maybe some potential that the read load could push a weak drive over the edge to failure. On my personal servers, application load is nearly meaningless, so I do an about monthly scrub from cron which I think will only scrub one zpool at a time per machine, which seems reasonable enough to me. I run relatively large spinning disks, so if I scrubbed on a daily basis, the drives would spend most of the day scrubbing and that doesn't seem reasonable. I haven't run ZFS in a work environment... I'd have to really consider how the read load impacted the production load and if scrubbing with limits to reduce production impact would complete in a reasonable amount of time... I've run some systems that are essentially alwayd busy and if a scrub would take several months, I'd probably only scrub when other systems indicate a problem and I can take the machine out of rotation to examine it. If I had very high reliability needs or a long time to get replacement drives, I might scrub more often? If I was worried about power consumption, I might scrub less often (and also let my servers and drives go into standby). The article's recommendation to scan at least once every 4 months seems pretty reasonable, although if you have seriously offline disks, maybe once a year is more approachable. I don't think I'd push beyond that, lots of things don't like to sit for a year and then turn on correctly.
- atmosx 9mo ago
  Once a month is fine ("/etc/cron.monthly/zfs-scrub"): #!/bin/bash # # ZFS scrub script for monthly maintenance # Place in /etc/cron.monthly/zfs-scrub POOL="storage" TAG="zfs-scrub" # Log start logger -t "$TAG" -p user.notice "Starting ZFS scrub on pool: $POOL" # Run the scrub if /sbin/zpool scrub "$POOL"; then logger -t "$TAG" -p user.notice "ZFS scrub initiated successfully on pool: $POOL" else logger -t "$TAG" -p user.err "Failed to start ZFS scrub on pool: $POOL" exit 1 fi exit 0
  - chungy 9mo ago
    That script might do with the "-w" parameter passed to scrub. Then "zpool scrub" won't return until the scrub is finished.
  - k_bx 9mo ago
    Didn't know about the logger script, looks nice. Can it wrap the launch of the scrub itself so that it logs like logger too, or do you separately track its stdout/stderr when something happens? update: figured how you can improve that call to add logs to logger
    - nubinetwork 9mo ago
      Scrub doesn't log anything by default, you run it and it returns quickly... you have to get the results out of zpool status or through zed.
    - atmosx 9mo ago
      The status of the pool is tracked by Prometheus though an ZFS exporter and faults are reported via alert manager & pushover.
- kanbankaren 9mo ago
  Once a month might be too high because HDDs are rated at ~ 180 TB workload/year. Remember, the workload/year limit includes read & writes and doesn't vary much by capacity, so a 10 TB HDD scrubbed monthly consumes 67% of the workload, let alone any other usage. Scrubbing every quarter is usually sufficient without putting high wear on the HDD.
  - Hakkin 9mo ago
    A scrub only reads allocated space, so in your 10TB example, a scrub would only read whatever portion of that 10TB is actually occupied by data. It's also usually recommended to keep your usage below 80% of the total pool size to avoid performance issues, so the worst case in your scenario would be more like ~53% assuming you follow the 80% rule.
    - formerly_proven 9mo ago
      Is the 80% rule real or just passed down across decades like other “x% free” rules? Those waste enormous amounts of resources on modern systems and I kind of doubt ZFS actually needs a dozen terabytes or more of free space in order to not shit the bed. Just like Linux doesn’t actually need >100 GB of free memory to work properly.
      - cornonthecobra 9mo ago
        Speaking strictly about ZFS internal operations, the free space requirement is closer to 5% on current ZFS versions. That allows for CoW and block reallocations in real-world pools. Heavy churn and very large files will increase that margin.
      - barrkel 9mo ago
        In practice you see noticeable degradation of performance for streaming reads of large files written after 85% or so. Files you used to be able to expect to get 500+MB/sec could be down to 50MB/sec. It's fragmentation, and it's fairly scale invariant, in my experience.
      - 9mo ago
- barrkel 9mo ago
  I scrub once a quarter because scrubs take 11 days to complete. I have 8x 18TB raidz2 pool, and I keep a couple of spare drives on hand so I can start a resilver as soon as an issue crops up. In the past, I've gone for a few years between scrubs. One system had a marginal I/O setup and was unreliable for high streaming load. When copying the pool off of it, I had to throttle the I/O to keep it reliable. No data loss though. Scrubs are intensive. They will IMO provoke failure in drives sooner than not doing them. But they're the kind of failures you want to bring forward if you can afford the replacements (and often the drives are under warranty anyway). If you don't scrub, eventually you generally start seeing one of two things: delays in reads and writes because drive error recovery is reading and rereading to recover data; or, if you have that disk behaviour disabled via firmware flags (and you should, unless you're reslivering and on your last disk of redundancy), you see zfs kicking a drive out of the pool during normal operations. If I start seeing unrecoverable errors, or a drive dropping out of the pool, I'll disable scrubs if I don't have a spare drive on hand to start mirroring straight away. But it's better to have the spares. At least two, because often a second drive shows weakness during resilver. There is a specific failure mode that scrubs defend against: silent disk corruption that only shows up when you read a file, but for files you almost never read. This is a pretty rare occurrence - it's never happened to me in about 50 drives worth of pools over 15 years or so. The way I think about this is, how is it actionable? If it's not a failing disk, you need to check your backups. And thus your scrub interval should be tied to your backup retention.
thatcks 9mo ago
The article is correct but it downplays an important limitation of ZFS scrubs when it talks about how they're different from fsck and chkdsk. As the article says (in different words), ZFS scrubs do not check filesystem objects for correctness and consistency; it only checks that they have the expected checksum and so have not become corrupted due to disk errors or other problems. Unfortunately it's possible for ZFS bugs and issues to give you filesystem objects that have problems, and as it stands today ZFS doesn't have anything that either checks or corrects these. Sometimes you find them through incorrect results; sometimes you discover they exist through ZFS assertion failures triggering kernel panics. (We run ZFS in production and have not been hit by these issues, at least not that we know about. But I know of some historical ZFS bugs in this area and mysterious issues that AFAIK have never been fully diagnosed.)
- mustache_kimono 9mo ago
  "Scrubs differ significantly from traditional filesystem checks. Tools such as fsck or chkdsk examine logical structures and attempt to repair inconsistencies related to directory trees, allocation maps, reference counts, and other metadata relationships. ZFS does not need to perform these operations during normal scrubs because its transactional design ensures metadata consistency. Every transaction group moves the filesystem from one valid state to another. The scrub verifies the correctness of the data and metadata at the block level, not logical relationships." > ZFS scrubs do not check filesystem objects for correctness and consistency; it only checks that they have the expected checksum and so have not become corrupted due to disk errors or other problems A scrub literally reads the object from disk. And, for each block, the checksums are read up the tree. The object is therefore guaranteed to be correct and consistent at least re: the tree of blocks written. > Unfortunately it's possible for ZFS bugs and issues to give you filesystem objects that have problems Can you give a more concrete example of what you mean? It sounds like you have some experience with ZFS, but "ZFS doesn't have an fsck" is also some truly ancient FUD, so you will forgive my skepticism. I'm willing to believe that you request an object and ZFS cannot return that object because of ... a checksum error or a read error in a single disk configuration, but what I have never seen is a scrub that indicates everything is fine, and then reads which don't return an object (because scrubs are just reads themselves?). Now, are things like pool metadata corruption possible in ZFS? Yes, certainly. I'm just not sure fsck would or could help you out of the same jam if you were using XFS or ext4. AFAIK fsck may repair inconsistencies but I'm not sure it can repair metadata any better than ZFS can?
klempner 9mo ago
>HDDs typically have a BER (Bit Error Rate) of 1 in 1015, meaning some incorrect data can be expected around every 100 TiB read. That used to be a lot, but now that is only 3 or 4 full drive reads on modern large-scale drives. Silent corruption is one of those problems you only notice after it has already done damage. While the advice is sound, this number isn't the right number for this argument. That 10^15 number is for UREs, which aren't going to cause silent data corruption -- simple naive RAID style mirroring/parity will easily recover from a known error of this sort without any filesystem layer checksumming. The rates for silent errors, where the disk returns the wrong data that benefit from checksumming, are a couple of orders of magnitude lower.
- iberator 9mo ago
  This is pure theory. Ber shouldn't be counted per sector etc? We shouldn't tread all disk space as single entity, IMO
  - thfuran 9mo ago
    Why would that make a difference unless some sectors have higher/lower error rates than others?
    - Dylan16807 9mo ago
      For a fixed bit error rate, making your typical error 100x bigger means it will happen 100x less often. If the typical error is an entire sector, that's over 30 thousand bits. 1:1e15 BER could mean 1 corrupted bit every 100 terabytes or it could mean 1 corrupted sector every 4 exabytes. Or anything in between. If there's any more detailed spec for what that number means, I'd love to see it.
- digiown 9mo ago
  This stat is also complete bullshit. If it were true, your scrubs of any 20+TB pool would get at least corrected errors quite frequently. But this is not the case. The consumer grade drives are often given an even lower spec of 1 in 1e14. For a 20TB drive, that's more than one error every scrub, which does not happen. I don't know about you, but I would not consider a drive to be functional at all if reading it out in full would produce more than one error on average. Pretty much nothing said on that datasheet reflects reality.
kalleboo 9mo ago
> HDDs typically have a BER (Bit Error Rate) of 1 in 1015, meaning some incorrect data can be expected around every 100 TiB read. That used to be a lot, but now that is only 3 or 4 full drive reads on modern large-scale drives I remember this argument way back 16 years ago when the "Why RAID 5 stops working in 2009" article[0] blew up. It's BS. Those aren't the actual average error rates. Those are inflated error rates below which the manufacturer does not want to bother supplying a warranty for. I have a pool with 260 TB worth of 10/14 TB disks in it 80% full, with monthly scrubs going back years. Not a single checksum error, and in total something like 30 reallocated sectors seen in SMART (half of those on a 7 year old drive). [0] https://www.zdnet.com/article/why-raid-5-stops-working-in-2009/ https://www.zdnet.com/article/why-raid-5-stops-working-in-20...
- mnw21cam 9mo ago
  Agreed. I have a couple of servers each with 168 hard drives, about 6 years old. A few hard drives are starting to fail. ZFS counts read errors (the drive reported an error because its checksum didn't match) and checksum errors (the drive returned data that was actually incorrect and ZFS caught it with a checksum). I have seen lots of read errors, but not a single checksum error yet. Though these are server-grade drives, which might be better than consumer-grade.

93 points by zdw 10mo ago

>Most systems that include ZFS schedule scrubs once per month. This frequency is appropriate for many environments, but high churn systems may require more frequent scrubs. Is there a more specific 'rule of thumb' for scrub frequency? What variables should one consider?

nubinetwork 9mo ago
Total pool size and speed. Less data scrubs faster, as do faster disks or disk topology (a 3 way stripe of nvme will scrub faster than a single sata ssd) For what its worth, I scrub daily mostly because I can. It's completely overkill, but if it only takes half an hour, then it can run in the middle of the night while I'm sleeping.
ssl-3 9mo ago
The cost of a scrub is just a flurry of disk reads and a reduction in performance during a scrub. If this cost is affordable on a daily basis, then do a scrub daily. If it's only affordable less often, then do it less often. (Whatever the case: It's not like a scrub causes any harm to the hardware or the data. It can run as frequently as you elect to tolerate.)
- agapon 9mo ago
  With HDDs, it's also mechanical wear and increased chance of a failure. SSDs are not fully immune to increased load either.
  - ssl-3 9mo ago
    Is there any evidence that suggests that reading from a hard drive (instead of it just spinning idle) increases physical wear in any meaningful way? Likewise, is there any evidence of this for solid-state storage?
    - rcxdude 9mo ago
      Yes. Hard drives have published "Annualized Workload Rate" ratings, which are in TB/year, and the manufacturers state there is no difference between reads and writes for the purpose of this rating. (https://www.toshiba-storage.com/trends-technology/mttf-what-hard-drive-reliability-really-means/ https://www.toshiba-storage.com/trends-technology/mttf-what-...) For SSDs, writes matter a lot more. Reads may increase the temperature of the drive, so they'll have some effect, but I don't think I've seen a read endurance rating for an SSD.
    - digiown 9mo ago
      Reading from it requires the read head to move, as opposed to spinning idle where the heads are parked on the side. Moving parts generally wear out over time.
toast0 9mo ago
Once a month seems like a reasonable rule of thumb. But you're balancing the cost of the scrub vs the benefit of learning about a problem as soon as possible. A scrub does a lot of I/O and a fair amount of computing. The scrub load competes with your application load and depending on the size of your disk(s) and their read bandwidth, it may take quite some time to do the scrub. There's even maybe some potential that the read load could push a weak drive over the edge to failure. On my personal servers, application load is nearly meaningless, so I do an about monthly scrub from cron which I think will only scrub one zpool at a time per machine, which seems reasonable enough to me. I run relatively large spinning disks, so if I scrubbed on a daily basis, the drives would spend most of the day scrubbing and that doesn't seem reasonable. I haven't run ZFS in a work environment... I'd have to really consider how the read load impacted the production load and if scrubbing with limits to reduce production impact would complete in a reasonable amount of time... I've run some systems that are essentially alwayd busy and if a scrub would take several months, I'd probably only scrub when other systems indicate a problem and I can take the machine out of rotation to examine it. If I had very high reliability needs or a long time to get replacement drives, I might scrub more often? If I was worried about power consumption, I might scrub less often (and also let my servers and drives go into standby). The article's recommendation to scan at least once every 4 months seems pretty reasonable, although if you have seriously offline disks, maybe once a year is more approachable. I don't think I'd push beyond that, lots of things don't like to sit for a year and then turn on correctly.
atmosx 9mo ago
Once a month is fine ("/etc/cron.monthly/zfs-scrub"): #!/bin/bash # # ZFS scrub script for monthly maintenance # Place in /etc/cron.monthly/zfs-scrub POOL="storage" TAG="zfs-scrub" # Log start logger -t "$TAG" -p user.notice "Starting ZFS scrub on pool: $POOL" # Run the scrub if /sbin/zpool scrub "$POOL"; then logger -t "$TAG" -p user.notice "ZFS scrub initiated successfully on pool: $POOL" else logger -t "$TAG" -p user.err "Failed to start ZFS scrub on pool: $POOL" exit 1 fi exit 0
- chungy 9mo ago
  That script might do with the "-w" parameter passed to scrub. Then "zpool scrub" won't return until the scrub is finished.
- k_bx 9mo ago
  Didn't know about the logger script, looks nice. Can it wrap the launch of the scrub itself so that it logs like logger too, or do you separately track its stdout/stderr when something happens? update: figured how you can improve that call to add logs to logger
  - nubinetwork 9mo ago
    Scrub doesn't log anything by default, you run it and it returns quickly... you have to get the results out of zpool status or through zed.
  - atmosx 9mo ago
    The status of the pool is tracked by Prometheus though an ZFS exporter and faults are reported via alert manager & pushover.
kanbankaren 9mo ago
Once a month might be too high because HDDs are rated at ~ 180 TB workload/year. Remember, the workload/year limit includes read & writes and doesn't vary much by capacity, so a 10 TB HDD scrubbed monthly consumes 67% of the workload, let alone any other usage. Scrubbing every quarter is usually sufficient without putting high wear on the HDD.
- Hakkin 9mo ago
  A scrub only reads allocated space, so in your 10TB example, a scrub would only read whatever portion of that 10TB is actually occupied by data. It's also usually recommended to keep your usage below 80% of the total pool size to avoid performance issues, so the worst case in your scenario would be more like ~53% assuming you follow the 80% rule.
  - formerly_proven 9mo ago
    Is the 80% rule real or just passed down across decades like other “x% free” rules? Those waste enormous amounts of resources on modern systems and I kind of doubt ZFS actually needs a dozen terabytes or more of free space in order to not shit the bed. Just like Linux doesn’t actually need >100 GB of free memory to work properly.
    - cornonthecobra 9mo ago
      Speaking strictly about ZFS internal operations, the free space requirement is closer to 5% on current ZFS versions. That allows for CoW and block reallocations in real-world pools. Heavy churn and very large files will increase that margin.
    - barrkel 9mo ago
      In practice you see noticeable degradation of performance for streaming reads of large files written after 85% or so. Files you used to be able to expect to get 500+MB/sec could be down to 50MB/sec. It's fragmentation, and it's fairly scale invariant, in my experience.
    - 9mo ago
barrkel 9mo ago
I scrub once a quarter because scrubs take 11 days to complete. I have 8x 18TB raidz2 pool, and I keep a couple of spare drives on hand so I can start a resilver as soon as an issue crops up. In the past, I've gone for a few years between scrubs. One system had a marginal I/O setup and was unreliable for high streaming load. When copying the pool off of it, I had to throttle the I/O to keep it reliable. No data loss though. Scrubs are intensive. They will IMO provoke failure in drives sooner than not doing them. But they're the kind of failures you want to bring forward if you can afford the replacements (and often the drives are under warranty anyway). If you don't scrub, eventually you generally start seeing one of two things: delays in reads and writes because drive error recovery is reading and rereading to recover data; or, if you have that disk behaviour disabled via firmware flags (and you should, unless you're reslivering and on your last disk of redundancy), you see zfs kicking a drive out of the pool during normal operations. If I start seeing unrecoverable errors, or a drive dropping out of the pool, I'll disable scrubs if I don't have a spare drive on hand to start mirroring straight away. But it's better to have the spares. At least two, because often a second drive shows weakness during resilver. There is a specific failure mode that scrubs defend against: silent disk corruption that only shows up when you read a file, but for files you almost never read. This is a pretty rare occurrence - it's never happened to me in about 50 drives worth of pools over 15 years or so. The way I think about this is, how is it actionable? If it's not a failing disk, you need to check your backups. And thus your scrub interval should be tied to your backup retention.

thatcks 9mo ago

The article is correct but it downplays an important limitation of ZFS scrubs when it talks about how they're different from fsck and chkdsk. As the article says (in different words), ZFS scrubs do not check filesystem objects for correctness and consistency; it only checks that they have the expected checksum and so have not become corrupted due to disk errors or other problems. Unfortunately it's possible for ZFS bugs and issues to give you filesystem objects that have problems, and as it stands today ZFS doesn't have anything that either checks or corrects these. Sometimes you find them through incorrect results; sometimes you discover they exist through ZFS assertion failures triggering kernel panics. (We run ZFS in production and have not been hit by these issues, at least not that we know about. But I know of some historical ZFS bugs in this area and mysterious issues that AFAIK have never been fully diagnosed.)

mustache_kimono 9mo ago
"Scrubs differ significantly from traditional filesystem checks. Tools such as fsck or chkdsk examine logical structures and attempt to repair inconsistencies related to directory trees, allocation maps, reference counts, and other metadata relationships. ZFS does not need to perform these operations during normal scrubs because its transactional design ensures metadata consistency. Every transaction group moves the filesystem from one valid state to another. The scrub verifies the correctness of the data and metadata at the block level, not logical relationships." > ZFS scrubs do not check filesystem objects for correctness and consistency; it only checks that they have the expected checksum and so have not become corrupted due to disk errors or other problems A scrub literally reads the object from disk. And, for each block, the checksums are read up the tree. The object is therefore guaranteed to be correct and consistent at least re: the tree of blocks written. > Unfortunately it's possible for ZFS bugs and issues to give you filesystem objects that have problems Can you give a more concrete example of what you mean? It sounds like you have some experience with ZFS, but "ZFS doesn't have an fsck" is also some truly ancient FUD, so you will forgive my skepticism. I'm willing to believe that you request an object and ZFS cannot return that object because of ... a checksum error or a read error in a single disk configuration, but what I have never seen is a scrub that indicates everything is fine, and then reads which don't return an object (because scrubs are just reads themselves?). Now, are things like pool metadata corruption possible in ZFS? Yes, certainly. I'm just not sure fsck would or could help you out of the same jam if you were using XFS or ext4. AFAIK fsck may repair inconsistencies but I'm not sure it can repair metadata any better than ZFS can?

klempner 9mo ago

>HDDs typically have a BER (Bit Error Rate) of 1 in 1015, meaning some incorrect data can be expected around every 100 TiB read. That used to be a lot, but now that is only 3 or 4 full drive reads on modern large-scale drives. Silent corruption is one of those problems you only notice after it has already done damage. While the advice is sound, this number isn't the right number for this argument. That 10^15 number is for UREs, which aren't going to cause silent data corruption -- simple naive RAID style mirroring/parity will easily recover from a known error of this sort without any filesystem layer checksumming. The rates for silent errors, where the disk returns the wrong data that benefit from checksumming, are a couple of orders of magnitude lower.

iberator 9mo ago
This is pure theory. Ber shouldn't be counted per sector etc? We shouldn't tread all disk space as single entity, IMO
- thfuran 9mo ago
  Why would that make a difference unless some sectors have higher/lower error rates than others?
  - Dylan16807 9mo ago
    For a fixed bit error rate, making your typical error 100x bigger means it will happen 100x less often. If the typical error is an entire sector, that's over 30 thousand bits. 1:1e15 BER could mean 1 corrupted bit every 100 terabytes or it could mean 1 corrupted sector every 4 exabytes. Or anything in between. If there's any more detailed spec for what that number means, I'd love to see it.
digiown 9mo ago
This stat is also complete bullshit. If it were true, your scrubs of any 20+TB pool would get at least corrected errors quite frequently. But this is not the case. The consumer grade drives are often given an even lower spec of 1 in 1e14. For a 20TB drive, that's more than one error every scrub, which does not happen. I don't know about you, but I would not consider a drive to be functional at all if reading it out in full would produce more than one error on average. Pretty much nothing said on that datasheet reflects reality.

kalleboo 9mo ago

> HDDs typically have a BER (Bit Error Rate) of 1 in 1015, meaning some incorrect data can be expected around every 100 TiB read. That used to be a lot, but now that is only 3 or 4 full drive reads on modern large-scale drives I remember this argument way back 16 years ago when the "Why RAID 5 stops working in 2009" article[0] blew up. It's BS. Those aren't the actual average error rates. Those are inflated error rates below which the manufacturer does not want to bother supplying a warranty for. I have a pool with 260 TB worth of 10/14 TB disks in it 80% full, with monthly scrubs going back years. Not a single checksum error, and in total something like 30 reallocated sectors seen in SMART (half of those on a 7 year old drive). [0] https://www.zdnet.com/article/why-raid-5-stops-working-in-2009/ https://www.zdnet.com/article/why-raid-5-stops-working-in-20...

mnw21cam 9mo ago
Agreed. I have a couple of servers each with 168 hard drives, about 6 years old. A few hard drives are starting to fail. ZFS counts read errors (the drive reported an error because its checksum didn't match) and checksum errors (the drive returned data that was actually incorrect and ZFS caught it with a checksum). I have seen lots of read errors, but not a single checksum error yet. Though these are server-grade drives, which might be better than consumer-grade.