[Users] discard support for SSD in OpenVZ kernel

[spameden]

Hi.


2013/8/27 David Brown <david at westcontrol.com>

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

I tend to disagree with this.

On the writing operations I saw at least 2x speed with discard / TRIM
enabled on dm-crypt LUKS partition.

On the another note it's not secure to use TRIM on dm-crypt, because
attacker might identify which blocks are free and potentially can detect
filesystem.

The fix for dm-crypt devices only will be in 3.1.x mainline, so don't think
it's gonna be ported in the current 2.6.32 branch.




> 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.
>

Well, if you won't use TRIM, you'll see in the future write performance
degraded all because of the SSD internals.


> 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.
>

I read about it, but don't think it's a proper solution, I'm thinking about
trying another virtualization technology instead.

LXC seems to be unstable for production yet.

Thinking about trying XEN on the more recent kernel.


>
> 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
> >
> >
> >
> >
> >
> > _______________________________________________
> > Users mailing list
> > Users at openvz.org
> > https://lists.openvz.org/mailman/listinfo/users
> >
>
>
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