On 12/15/2010 03:26 PM, luvar@xxxxxxxxxxxx wrote:
Hi,
Iam new in problematic of caching persistent data. I want to use caching (on ssd disk) on my desktop with a lot of ram. I hope, it will provide me better latency for starting programs (developer/scientific desktop). My questions are:
Is it possible to use single ssd disk for making two caches? One for /dev/md2 (root partition, raid10) and second for /dev/md3 (raid5,home partition, etc) (I hope this is possible by partitioning that ssd disk and using partitions for caching)
Yep. One cache device can be used with (currently) up to 256 backing
devices.
Is it dangerous (in terms of data loss) to use writethrough caching to single ssd disk of raid5 block device? (I have 5 disks, using lvm on top of raid).
Shouldn't be, recovery from unclean shutdown is quite well tested, both
writethrough and writeback should be perfectly safe.
For caching device is most critical number for random reads or operations per second. Is it right?
For writethrough caching random reads, for writeback both reads and writes.
Trim is not need feature for cache ssd device. It will have same cache performance without it. Is it right?
Really just depends on the ssd; some (cheaper, earlier) drives are known
for performance significantly degrading over time, with trim helps with
(but doesn't necessarily eliminate). Bcache uses trim if it's available.
What benefits/disadvantages have bcache [1] project over dm-cache [2] and flashcache [3]? My current knowledge is that writethrough is possible only in bcache, but dm-cache is implemented in more "standard" way (thdough devicemaper). Also bcache has some more information on web.
[1] - http://bcache.evilpiepirate.org/
[2] - http://users.cis.fiu.edu/~zhaom/dmcache/index.html
[3] - https://github.com/facebook/flashcache
Flashcache is based off of dm-cache.
Flashcache has been used in production awhile, bcache is still a little
rough around the edges - but bcache has better performance, more
features, and it always orders writes correctly so as to be crash safe
(flashcache has a "torn write" problem).
PS: Is there any tutorial for gentoo users?
Nah, I'm an ubuntu/debian user. For caching / the important thing is to
hook into your initramfs and get everything loaded before you mount your
root filesystem.
Thanks for any explanations / answers,
--
LuVar
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