[Users] discard support for SSD in OpenVZ kernel

[David Brown]

The answer is quite simple - don't use "discard" mounts.  They make your
SSD much slower, especially for metadata-heavy operations.

The problem is that the halfwits that added TRIM to the SATA
specifications made it a synchronous operation, so it blocks all queues
and buffers.  They also failed to give it proper semantics, such as
specifying that reading from a TRIM'ed sector would give all zeros.  If
the people behind this nonsense had read the SCSI specifications for the
equivalent operation, they would have made a much better TRIM that was
asynchronous, could be queued, and would guarantee reading all zeros
from a TRIM'ed block - which would have been much more useful.

Off-line TRIM using fstrim is useful, but not essential if you have
bought a half-decent SSD that is not too small, and not too old.  So use
fstrim if it works on your setup - but don't worry if it doesn't.  It's
very unlikely that you will notice the difference.

Hope that helps,

David



On 27/08/13 17:10, spameden wrote:
> is it implemented?
> 
> I've tried on 3.2.0-4 debian wheezy default kernel it's working just fine:
> # dmsetup table
> vg0-home: 0 443277312 linear 9:1 25166208
> home: 0 443273216 crypt aes-cbc-plain
> 0000000000000000000000000000000000000000000000000000000000000000 0 253:2
> 4096 1 allow_discards
> 
> But not on OpenVZ's 2.6.32.xxxx:
> # dmsetup table
> vg0-home: 0 443277312 linear 9:1 25166208
> home: 0 443273216 crypt aes-cbc-plain
> 0000000000000000000000000000000000000000000000000000000000000000 0 253:2
> 4096
> vg0-swap: 0 4194304 linear 9:1 20971904
> vg0-root: 0 20971520 linear 9:1 384
> 
> 
> 
> 
> 
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