RE: [PATCH] brd: Refuse improperly aligned discard requests

[Elliott, Robert (Persistent Memory)]

> -----Original Message-----
> From: linux-kernel-owner@xxxxxxxxxxxxxxx [mailto:linux-kernel-
> owner@xxxxxxxxxxxxxxx] On Behalf Of Jan Kara
> Sent: Wednesday, November 4, 2015 10:14 AM
> To: axboe@xxxxxxxxx
> Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>; linux-fsdevel@xxxxxxxxxxxxxxx;
> Christoph Hellwig <hch@xxxxxxxxxxxxx>; Jan Kara <jack@xxxxxxxx>
> Subject: [PATCH] brd: Refuse improperly aligned discard requests
> 
> Currently when improperly aligned discard request is submitted, we just
> silently discard more / less data which results in filesystem corruption
> in some cases. Refuse such misaligned requests.

I agree discarding more than requested is very bad.

If they are routed to SCSI or ATA devices, though, the discard commands
(SCSI UNMAP or ATA DATA SET MANAGEMENT/TRIM) are just hints, so there 
is no guarantee the discard will do anything.  Are you finding 
filesystems that still don't understand that?   dm-raid held that
mistaken assumption for a long time.

---
Robert Elliott, HP Server Storage

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