On 8/29/20 12:02 AM, Roman Mamedov wrote:
On Fri, 28 Aug 2020 22:08:22 -0500
"R. Ramesh" <rramesh@xxxxxxxxxxx> wrote:
I do not know how SSD caching is implemented. I assumed it will be
somewhat similar to memory cache (L2 vs L3 vs L4 etc). I am hoping that
with SSD caching, reads/writes to disk will be larger in size and
sequential within a file (similar to cache line fill in memory cache
which results in memory bursts that are efficient). I thought that is
what SSD caching will do to disk reads/writes. I assumed, once reads
(ahead) and writes (assuming writeback cache) buffers data sufficiently
in the SSD, all reads/writes will be to SSD with periodic well organized
large transfers to disk. If I am wrong here then I do not see any point
in SSD as a cache. My aim is not to optimize by cache hits, but optimize
by preventing disks from thrashing back and forth seeking after every
block read. I suppose Linux (memory) buffer cache alleviates some of
that. I was hoping SSD will provide next level. If not, I am off in my
understanding of SSD as a disk cache.
Just try it, as I said before with LVM it is easy to remove if it doesn't work
out. You can always go to the manual copying method or whatnot, but first why
not check if the automatic caching solution might be "good enough" for your
needs.
Yes it usually tries to avoid caching long sequential reads or writes, but
there's also quite a bit of other load on the FS, i.e. metadata. I found that
browsing directories and especially mounting the filesystem had a great
benefit from caching.
You are correct that it will try to increase performance via writeback
caching, however with LVM that needs to be enabled explicitly:
https://www.systutorials.com/docs/linux/man/7-lvmcache/#lbAK
And of course a failure of that cache SSD will mean losing some data, even if
the main array is RAID. Perhaps should consider a RAID of SSDs for cache in
that case then.
Yes, I have 2x500GB ssds for cache. May be, I should do raid1 on them
and use as cache volume.
I thought SSDs are more reliable and even when they begin to die, they
become readonly before quitting. Of course, this is all theory, and I
do not think standards exists on how they behave when reaching EoL.
Ramesh
