On Wed, 13 Jan 2016 08:43:28 PM Craig Sanders via luv-main wrote:
On Wed, Jan 13, 2016 at 01:28:57PM +1100, Trent W. Buck wrote:
AFAICT the only reason SSHDs exist are: * Windows has nothing like bcache/l2arc; or
it does. it's called ReadyBoost. Compared to bcache/flashcache or L2ARC for ZFS, it sucks. You can't just tell Windows to use an SSD (or partition thereof) to cache an arbitrary disk....well, in theory you can but in practice Windows itself decides whether that option will be available by its own inscrutable and undocumented method.
https://en.wikipedia.org/wiki/ReadyBoost ReadyBoost is described as supporting USB Flash, CF, and other storage systems. USB Flash is notoriously unreliable and has given flash storage a bad reputation. It seems to be only read caching, it would be ridiculous to consider devices like USB Flash for write-back caching.
I discovered this just last week after upgrading my win7 games box to have an SSD as a boot disk, decided to try ReadyBoost for my main 2TB steam library drive (not an SSHD) so made a 40GB partition for it. No matter what I tried, I couldn't get windows to make the option available in its Disk Manager GUI - it was there, just greyed out.
As you noted later in your message SSD storage in the TB range is getting quite affordable now. If you are buying an expensive gaming PC then getting 1TB of SSD for all your favourite games isn't going to change the price much. 2TB of SSD is in the range that many middle class people are prepared to pay. There doesn't seem to be a good case for MS to invest in further development of ReadyBoost. I don't like the idea of Bcache. What I would like to do is to be able to tell BTRFS to use certain disks for metadata only. For example I'd like to have a server with the root and /home subvols on a RAID-1 pair of SSDs as well as the metadata for all other subvols. Then have all the bulk data on a pair of large SATA disks. For my home fileserver I have no performance problems with tasks such as NFS serving big MP4 files (like my collection of FullHD music videos downloaded from Youtube), but I do have performance problems with ls. Also my home server has backups of a number of other systems (including the LUV server). When I run rsync without the -c option metadata access is a major bottleneck.
Microsoft seems to have abandoned the ReadyBoost idea. and they seem to be confused about what SSD caching of hard disks is good for - their documentation is all about how it might help on low-memory machines (<4GB), presumably because they don't have enough RAM to cache large chunks disk in RAM. as bcache etc on linux prove, SSD caching is beneficial even on machines with lots of RAM.
It really depends on what you are doing. A SSD read cache provides no benefits if the working set of the applications that are running fits in RAM once the system has been running for a while. For example the LUV 524M of storage for all MySQL databases and probably less than 100M of other data that is used often. It has 2.5G of RAM so it does hardly any disk access apart from synchronous writes for the mail queue once it's been running for a while.
windows is so damn primitive and restricted. i can't understand how anyone could possibly think it's any good or that it makes a decent desktop environment. it's almost unusable in its awful crappiness. i'm glad i only have to use it as a games launcher.
Some people think of a desktop OS as a games launcher, in which case Windows is doing ok in terms of game support. Steam is moving towards Linux which significantly changes things but at the moment Windows is a passable game launcher.
interestingly, i don't even have to use my KVM to switch to the windows box any more if i don't want to - steam's in-home streaming feature means that in addition to running native linux games on steam, i can "stream" windows-only games from my win7 box. that means it executes on windows but displays on my linux box in either full-screen or windowed mode. would probably suck on wireless but there's no noticable lag or performance loss on a wired gigabit network. i'm kind of amazed at how well it works.
That's interesting.
but given that the SSHDs i bought were roughly the same price as non-SSHDs, i don't see any harm and some potential benefit in using them on linux. 8GB cache isn't much but it "just works" without any hassle or configuration. it's giving me some SSD caching on my ZFS 'backup' pool without having to add another SSD to the same andr dedicate an SSD partition to the task.
For a filesystem like ZFS or BTRFS where the read hot spots move around the benefit of a small amount of read caching is probably small. If they do write caching then that could make a huge difference as the seek patterns for ZFS and BTRFS can cause significant problems.
but if you're not in a hurry to get an SSD, rumour has it that there will be significant increases in capacity AND reductions in price on SSDs this year.
Just like every other year. ;) -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/