6 ms·

kind-of related, is anyone using bcache with linux yet? how easy is it to get working? does it speed things up as expected? http://arstechnica.com/informatio

kind-of related, is anyone using bcache with linux yet? how easy is it to get working? does it speed things up as expected? http://arstechnica.com/information-technology/2013/07/linux-3-10-out-with-better-ssd-caching-and-radeon-support/ http://arstechnica.com/information-technology/2013/07/linux-...

Sami_Lehtinen 13y ago
Well, I've been using it for over an year now. Yes, it does really speed up things. I've found out that people grossly over estimate need for SSD space. I'm using 64 GB SSD with 3TB drive and hard disk is really rearely touched. I've even enabled power saving spin down for it, so I know when it starts. In normal daily usage HDD isn't being touched at all. It's only when something like linux updates are being run or so. I've also enabled write-back caching without maximum time limit. If you use write-through caching it naturally causes HDD to run all the time. I've chosen to cache everything, not only random reads. Because I have plenty of space with 64 GB SSD. I don't know what people are doing who claim thei need larger SSD than that. Maybe they're working with large data sets or have absolutely massive games or so. As summary, yes, I love it. SSD is never full, I'm not running out of disk space and yes, I do get pure SSD performance over 99% of the time. Only if I pickup some movies or music which has been around for months without being accessed then there's HDD access of course. Setup was ok, because I did it when I replaced my computer so I built everything from scratch anyway. I'm going to blog about that, but I have huge backlog of stuff to get blogged. P.S. Some cache vendors (like Seagate Hybrids drives) recommend using write-through caching, because in the case SSD dies, you'll still have fully working file system on HDD. With write-through caching, things are going to be very badly messed up if SSD dies. Practically totally unrecoverable situation. But that's why we got backups, right?
- chetanahuja 13y ago
  "With write-through caching, things are going to be very badly messed up if SSD dies" You probably meant the reverse of that (With write-back caching... etc.).
  - Sami_Lehtinen 13y ago
    Yep, true.
- wtallis 13y ago
  64GB SSDs aren't large enough to take full advantage of the wide data paths offered by modern SSD controllers that stripe accesses across many flash chips. Going up to at least 256GB is usually necessary to get the full speed possible from having every channel populated. http://www.anandtech.com/bench/product/752?vs=750 http://www.anandtech.com/bench/product/752?vs=750
  - justincormack 13y ago
    For most applications you wont notice
    - rektide 13y ago
      If you're building a server and spending money to put a SSD in front of hard drives because you want throughput, it behooves you to consider throughput/$. Maybe you'll notice, maybe you wont. There's a place for good enough, and any front-SSD cache will be a huge advance for sure over not. But I second the notion that you probably want to find a SSD that is performant, and low capacity ones often are heavily compromised in this department.
      - justincormack 13y ago
        Generally the software will bottleneck before the drive. Its pretty hard to get rated performance out of an SSD. You need to keep the queue at pretty much full depth, which means aio or (much harder) threading. Most real world software at present gets nothing like SSD potential performance in real world situations, as it is mostly build for HDD where none of this stuff mattered.

Sami_Lehtinen 13y ago

Well, I've been using it for over an year now. Yes, it does really speed up things. I've found out that people grossly over estimate need for SSD space. I'm using 64 GB SSD with 3TB drive and hard disk is really rearely touched. I've even enabled power saving spin down for it, so I know when it starts. In normal daily usage HDD isn't being touched at all. It's only when something like linux updates are being run or so. I've also enabled write-back caching without maximum time limit. If you use write-through caching it naturally causes HDD to run all the time. I've chosen to cache everything, not only random reads. Because I have plenty of space with 64 GB SSD. I don't know what people are doing who claim thei need larger SSD than that. Maybe they're working with large data sets or have absolutely massive games or so. As summary, yes, I love it. SSD is never full, I'm not running out of disk space and yes, I do get pure SSD performance over 99% of the time. Only if I pickup some movies or music which has been around for months without being accessed then there's HDD access of course. Setup was ok, because I did it when I replaced my computer so I built everything from scratch anyway. I'm going to blog about that, but I have huge backlog of stuff to get blogged. P.S. Some cache vendors (like Seagate Hybrids drives) recommend using write-through caching, because in the case SSD dies, you'll still have fully working file system on HDD. With write-through caching, things are going to be very badly messed up if SSD dies. Practically totally unrecoverable situation. But that's why we got backups, right?

chetanahuja 13y ago
"With write-through caching, things are going to be very badly messed up if SSD dies" You probably meant the reverse of that (With write-back caching... etc.).
- Sami_Lehtinen 13y ago
  Yep, true.
wtallis 13y ago
64GB SSDs aren't large enough to take full advantage of the wide data paths offered by modern SSD controllers that stripe accesses across many flash chips. Going up to at least 256GB is usually necessary to get the full speed possible from having every channel populated. http://www.anandtech.com/bench/product/752?vs=750 http://www.anandtech.com/bench/product/752?vs=750
- justincormack 13y ago
  For most applications you wont notice
  - rektide 13y ago
    If you're building a server and spending money to put a SSD in front of hard drives because you want throughput, it behooves you to consider throughput/$. Maybe you'll notice, maybe you wont. There's a place for good enough, and any front-SSD cache will be a huge advance for sure over not. But I second the notion that you probably want to find a SSD that is performant, and low capacity ones often are heavily compromised in this department.
    - justincormack 13y ago
      Generally the software will bottleneck before the drive. Its pretty hard to get rated performance out of an SSD. You need to keep the queue at pretty much full depth, which means aio or (much harder) threading. Most real world software at present gets nothing like SSD potential performance in real world situations, as it is mostly build for HDD where none of this stuff mattered.