From: Dave Chinner <david@fromorbit.com>
To: Sarthak Kukreti <sarthakkukreti@chromium.org>
Cc: dm-devel@redhat.com, linux-block@vger.kernel.org,
linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-fsdevel@vger.kernel.org, Jens Axboe <axboe@kernel.dk>,
Alasdair Kergon <agk@redhat.com>,
Mike Snitzer <snitzer@kernel.org>,
Christoph Hellwig <hch@infradead.org>,
Brian Foster <bfoster@redhat.com>, Theodore Ts'o <tytso@mit.edu>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Bart Van Assche <bvanassche@google.com>,
"Darrick J. Wong" <djwong@kernel.org>
Subject: Re: [PATCH v8 3/5] loop: Add support for provision requests
Date: Mon, 9 Oct 2023 10:37:52 +1100 [thread overview]
Message-ID: <ZSM9UDMFNs0050pr@dread.disaster.area> (raw)
In-Reply-To: <20231007012817.3052558-4-sarthakkukreti@chromium.org>
On Fri, Oct 06, 2023 at 06:28:15PM -0700, Sarthak Kukreti wrote:
> Add support for provision requests to loopback devices.
> Loop devices will configure provision support based on
> whether the underlying block device/file can support
> the provision request and upon receiving a provision bio,
> will map it to the backing device/storage. For loop devices
> over files, a REQ_OP_PROVISION request will translate to
> an fallocate mode 0 call on the backing file.
>
> Signed-off-by: Sarthak Kukreti <sarthakkukreti@chromium.org>
> Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Hmmmm.
This doesn't actually implement the required semantics of
REQ_PROVISION. Yes, it passes the command to the filesystem
fallocate() implementation, but fallocate() at the filesystem level
does not have the same semantics as REQ_PROVISION.
i.e. at the filesystem level, fallocate() only guarantees the next
write to the provisioned range will succeed without ENOSPC, it does
not guarantee *every* write to the range will succeed without
ENOSPC. If someone clones the loop file while it is in use (i.e.
snapshots it via cp --reflink) then all guarantees that the next
write to a provisioned LBA range will succeed without ENOSPC are
voided.
So while this will work for basic testing that the filesystem is
issuing REQ_PROVISION based IO correctly, it can't actually be used
for hosting production filesystems that need full REQ_PROVISION
guarantees when the loop device backing file is independently
shapshotted via FICLONE....
At minimuim, this set of implementation constraints needs tobe
documented somewhere...
-Dave.
--
Dave Chinner
david@fromorbit.com
next prev parent reply other threads:[~2023-10-08 23:37 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-10-07 1:28 [PATCH v8 0/5] Introduce provisioning primitives Sarthak Kukreti
2023-10-07 1:28 ` [PATCH v8 1/5] block: Don't invalidate pagecache for invalid falloc modes Sarthak Kukreti
2023-10-11 5:59 ` Christoph Hellwig
2023-10-07 1:28 ` [PATCH v8 2/5] block: Introduce provisioning primitives Sarthak Kukreti
2023-10-07 1:28 ` [PATCH v8 3/5] loop: Add support for provision requests Sarthak Kukreti
2023-10-08 23:37 ` Dave Chinner [this message]
2023-10-10 22:43 ` Sarthak Kukreti
2023-10-10 23:59 ` Dave Chinner
2023-10-07 1:28 ` [PATCH v8 4/5] dm: Add block provisioning support Sarthak Kukreti
2023-10-07 1:28 ` [PATCH v8 5/5] block: Pass unshare intent via REQ_OP_PROVISION Sarthak Kukreti
2023-10-08 23:27 ` Dave Chinner
2023-10-10 22:42 ` Sarthak Kukreti
2023-10-11 0:06 ` Dave Chinner
2023-10-08 23:41 ` [RFC PATCH 6/5] xfs: detect block devices requiring provisioning Dave Chinner
2023-10-08 23:45 ` [RFC PATCH 7/5] xfs: add block device provisioning for fallocate Dave Chinner
2023-10-08 23:50 ` [PATCH v8 0/5] Introduce provisioning primitives Dave Chinner
2023-10-10 22:42 ` Sarthak Kukreti
2023-10-11 0:13 ` Dave Chinner
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