6 ms·

I can't seem to dig this out but I vaguely remember some disks (HDD, not SSD) had this mode when data is: * acknowledged & buffered in the cache * not phy

I can't seem to dig this out but I vaguely remember some disks (HDD, not SSD) had this mode when data is: * acknowledged & buffered in the cache * not physically written to the plate (yet) * but still guaranteed to be written, even when power fails Anybody can confirm that?

zinekeller 5y ago
> acknowledged & buffered in the cache I remember some enterprise drives having a capacitor for this exact reason, although at the time some manufacturers (maybe it was Hitachi, can't remember exactly who) experimenting with using a flash storage (probably SLC) as the cache.
masklinn 5y ago
Some SSDs behave like this (well not with plates but you get the gist). The issue is others don’t have the 3rd bit, and you have to test to know which is which, unless it’s a server-grade ssd which advertises power loss protection (and even then you should probably test it).
actionfromafar 5y ago
There was a drive which used the rotational energy in the platters to generate electricity enough to write out what was in the cache. I wanna say it was IBM.
- mikequinlan 5y ago
  Possibly Western Digital. https://arstechnica.com/civis/viewtopic.php?f=2&t=1478947&start=120 https://arstechnica.com/civis/viewtopic.php?f=2&t=1478947&st...
bombcar 5y ago
Some hardware raid controllers do that with their battery backed cache - they return written before it’s flushed to disk because they have a battery in case of power interruption.
- WrtCdEvrydy 5y ago
  I always wondered... does the battery power the disk so the writes are done or does it just hold the data in the RAID controller until the computer is re-powered?
  - dijit 5y ago
    The second one. On power loss the drives will immediately lose power, they have little capacitors in them that allow the head to safely rest on power loss. (it used to be that the head just crashed into the spinning platter, can you even imagine). Part of the Server boot-up is to flush the raid caches, which are battery backed volatile storage. (and the batteries are fairly big so can last about 7 days in most cases). The battery being big doesn't matter to the drives though, there would be no guarantee that you could finish writing the cache to the drives with the capacity of the battery: since drives use a considerably larger amount of power than keeping some DIMMs hot.
    - bombcar 5y ago
      Exactly - the better controllers would "lock" and basically refuse to do anything until they saw the array they wanted to write to (which was always fun when dealing with a remaindered system pulled from production, often requiring a full BIOS reset or wipe on the card).
    - cout 5y ago
      > On power loss the drives will immediately lose power, they have little capacitors in them that allow the head to safely rest on power loss. (it used to be that the head just crashed into the spinning platter, can you even imagine). I remember as a kid running PARK.COM to park the heads before powering off the computer every night; I continued this habit for many years after I got my own computer, until I learned that hard drives now have mechanisms that make this unnecessary.
    - praptak 5y ago
      > it used to be that the head just crashed into the spinning platter, can you even imagine If I remember correctly, it was not that bad. The head being parked in a random place meant it could damage the plate if the disk was moved or shaken. I think that modern disks would indeed crash (if not for autoparking) but they have much smaller flying heights and thus require serious active magic to keep the head from hitting the plate.

zinekeller 5y ago

> acknowledged & buffered in the cache I remember some enterprise drives having a capacitor for this exact reason, although at the time some manufacturers (maybe it was Hitachi, can't remember exactly who) experimenting with using a flash storage (probably SLC) as the cache.

masklinn 5y ago

Some SSDs behave like this (well not with plates but you get the gist). The issue is others don’t have the 3rd bit, and you have to test to know which is which, unless it’s a server-grade ssd which advertises power loss protection (and even then you should probably test it).

actionfromafar 5y ago

There was a drive which used the rotational energy in the platters to generate electricity enough to write out what was in the cache. I wanna say it was IBM.

mikequinlan 5y ago
Possibly Western Digital. https://arstechnica.com/civis/viewtopic.php?f=2&t=1478947&start=120 https://arstechnica.com/civis/viewtopic.php?f=2&t=1478947&st...

bombcar 5y ago

Some hardware raid controllers do that with their battery backed cache - they return written before it’s flushed to disk because they have a battery in case of power interruption.

WrtCdEvrydy 5y ago
I always wondered... does the battery power the disk so the writes are done or does it just hold the data in the RAID controller until the computer is re-powered?
- dijit 5y ago
  The second one. On power loss the drives will immediately lose power, they have little capacitors in them that allow the head to safely rest on power loss. (it used to be that the head just crashed into the spinning platter, can you even imagine). Part of the Server boot-up is to flush the raid caches, which are battery backed volatile storage. (and the batteries are fairly big so can last about 7 days in most cases). The battery being big doesn't matter to the drives though, there would be no guarantee that you could finish writing the cache to the drives with the capacity of the battery: since drives use a considerably larger amount of power than keeping some DIMMs hot.
  - bombcar 5y ago
    Exactly - the better controllers would "lock" and basically refuse to do anything until they saw the array they wanted to write to (which was always fun when dealing with a remaindered system pulled from production, often requiring a full BIOS reset or wipe on the card).
  - cout 5y ago
    > On power loss the drives will immediately lose power, they have little capacitors in them that allow the head to safely rest on power loss. (it used to be that the head just crashed into the spinning platter, can you even imagine). I remember as a kid running PARK.COM to park the heads before powering off the computer every night; I continued this habit for many years after I got my own computer, until I learned that hard drives now have mechanisms that make this unnecessary.
  - praptak 5y ago
    > it used to be that the head just crashed into the spinning platter, can you even imagine If I remember correctly, it was not that bad. The head being parked in a random place meant it could damage the plate if the disk was moved or shaken. I think that modern disks would indeed crash (if not for autoparking) but they have much smaller flying heights and thus require serious active magic to keep the head from hitting the plate.