From: Maxim Levitsky <mlevitsk@redhat.com>
To: Nir Soffer <nsoffer@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Max Reitz <mreitz@redhat.com>,
qemu-block <qemu-block@nongnu.org>, Nir Soffer <nirsof@gmail.com>,
QEMU Developers <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH] block: posix: Always allocate the first block
Date: Mon, 26 Aug 2019 01:17:30 +0300 [thread overview]
Message-ID: <44a4d19bd9b7cc760a015570110eb27efe6e0055.camel@redhat.com> (raw)
In-Reply-To: <CAMRbyyt2PTWJBuR7Ni+gSCeHGpFuF-hkmCEG0Sqvm2gzFQL=CQ@mail.gmail.com>
On Sun, 2019-08-25 at 22:51 +0300, Nir Soffer wrote:
> On Sun, Aug 25, 2019 at 10:44 AM Maxim Levitsky <mlevitsk@redhat.com> wrote:
> > On Sat, 2019-08-17 at 00:21 +0300, Nir Soffer wrote:
> > > When creating an image with preallocation "off" or "falloc", the first
> > > block of the image is typically not allocated. When using Gluster
> > > storage backed by XFS filesystem, reading this block using direct I/O
> > > succeeds regardless of request length, fooling alignment detection.
> > >
> > > In this case we fallback to a safe value (4096) instead of the optimal
> > > value (512), which may lead to unneeded data copying when aligning
> > > requests. Allocating the first block avoids the fallback.
> > >
> > > When using preallocation=off, we always allocate at least one filesystem
> > > block:
> > >
> > > $ ./qemu-img create -f raw test.raw 1g
> > > Formatting 'test.raw', fmt=raw size=1073741824
> > >
> > > $ ls -lhs test.raw
> > > 4.0K -rw-r--r--. 1 nsoffer nsoffer 1.0G Aug 16 23:48 test.raw
> >
> > Are you sure about this?
>
> This is the new behaviour with this change...
>
> > [mlevitsk@maximlenovopc ~/work/test_area/posix-file 0]$ qemu-img create -f raw test.raw 1g -o preallocation=off
> > Formatting 'test.raw', fmt=raw size=1073741824 preallocation=off
> > [mlevitsk@maximlenovopc ~/work/test_area/posix-file 0]$ls -lhs ./test.raw
> > 0 -rw-r--r--. 1 mlevitsk mlevitsk 1.0G Aug 25 10:38 ./test.raw
> >
> > ext4, tested on qemu-4.0.0 and qemu git master.
>
> And this is the old behavior. I guess the commit message does not make it clear.
Ah, thanks!
> > From what I remember, the only case when posix-raw touches the first block is to zero it out
> > when running on top of kernel block device, to erase whatever header might be there, and this
> > is also kind of a backward compat hack which might be one day removed.
>
> This change is only for file, on block storage we use BLKSSZGET.
>
> > [...]
> >
> > Best regards,
> > Maxim Levitsky
> >
> >
Best regards,
Maxim Levitsky
prev parent reply other threads:[~2019-08-25 22:18 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-16 21:21 [Qemu-devel] [PATCH] block: posix: Always allocate the first block Nir Soffer
2019-08-16 21:57 ` [Qemu-devel] [Qemu-block] " John Snow
2019-08-16 22:45 ` Nir Soffer
2019-08-16 23:00 ` John Snow
2019-08-22 11:30 ` [Qemu-devel] " Nir Soffer
2019-08-22 14:28 ` Max Reitz
2019-08-22 16:39 ` Nir Soffer
2019-08-22 18:11 ` Max Reitz
2019-08-22 19:01 ` Nir Soffer
2019-08-23 13:58 ` Max Reitz
2019-08-23 16:30 ` Nir Soffer
2019-08-23 17:41 ` Max Reitz
2019-08-23 16:48 ` Nir Soffer
2019-08-23 17:53 ` Max Reitz
2019-08-24 22:57 ` Nir Soffer
2019-08-25 7:44 ` [Qemu-devel] [Qemu-block] " Maxim Levitsky
2019-08-25 19:51 ` Nir Soffer
2019-08-25 22:17 ` Maxim Levitsky [this message]
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