Re: about TRIM/DISCARD support and barriers

On Sun, 2008-11-23 at 05:35 -0700, Matthew Wilcox wrote:
The current VFAT filesystem implements DISCARD.

As does btrfs. I think I posted ext2 patches a little while ago too? Or
someone else did. Did they get merged?

Here's the patch I've
been testing -- it's not right; it causes SSDs from two different vendors
to hang. I'm waiting for a free slot on the SATA protocol analyser to
figure out what's we're doing wrong.

Note that the SCSI UNMAP command does not yet have an official number,
so this patch cannot yet be applied (... how much longer until ATA is no
longer part of SCSI?)

We don't attempt to put non-contiguous ranges into a single TRIM yet.

We don't even merge contiguous ranges -- I still need to fix the
elevators to stop writes crossing writes, before we can stop discards
from also being barriers. (Discards are just writes, for the purpose of
that conversation).

We currently assume all SCSI devices support UNMAP instead of checking
the feature flag (because last time I looked, I couldn't tell what flag
I was supposed to check). Once we do that, I need to set that flag in
libata-scsi depending on the support for the TRIM bit in the identify
command.

The code in drivers/ide does check the feature, although I'm not 100%
sure I got that part right.

--
David Woodhouse Open Source Technology Centre
David.Woodhouse@xxxxxxxxx Intel Corporation

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