On 19/04/2021 07.06, Thomas Huth wrote:
On 16/04/2021 22.34, Nir Soffer wrote:
On Fri, Apr 16, 2021 at 8:23 AM Thomas Huth <th...@redhat.com> wrote:
A customer reported that running
qemu-img convert -t none -O qcow2 -f qcow2 input.qcow2 output.qcow2
fails for them with the following error message when the images are
stored on a GPFS file system:
qemu-img: error while writing sector 0: Invalid argument
After analyzing the strace output, it seems like the problem is in
handle_aiocb_write_zeroes(): The call to fallocate(FALLOC_FL_PUNCH_HOLE)
returns EINVAL, which can apparently happen if the file system has
a different idea of the granularity of the operation. It's arguably
a bug in GPFS, since the PUNCH_HOLE mode should not result in EINVAL
according to the man-page of fallocate(), but the file system is out
there in production and so we have to deal with it. In commit 294682cc3a
("block: workaround for unaligned byte range in fallocate()") we also
already applied the a work-around for the same problem to the earlier
fallocate(FALLOC_FL_ZERO_RANGE) call, so do it now similar with the
PUNCH_HOLE call.
Signed-off-by: Thomas Huth <th...@redhat.com>
---
block/file-posix.c | 7 +++++++
1 file changed, 7 insertions(+)
diff --git a/block/file-posix.c b/block/file-posix.c
index 20e14f8e96..7a40428d52 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -1675,6 +1675,13 @@ static int handle_aiocb_write_zeroes(void *opaque)
}
s->has_fallocate = false;
} else if (ret != -ENOTSUP) {
+ if (ret == -EINVAL) {
+ /*
+ * File systems like GPFS do not like unaligned byte
ranges,
+ * treat it like unsupported (so caller falls back to
pwrite)
+ */
+ return -ENOTSUP;
This skips the next fallback, using plain fallocate(0) if we write
after the end of the file. Is this intended?
We can treat the buggy EINVAL return value as "filesystem is buggy,
let's not try other options", or "let's try the next option". Since falling
back to actually writing zeroes is so much slower, I think it is better to
try the next option.
I just did the same work-around as in commit 294682cc3a7 ... so if we agree
to try the other options, too, we should change that spot, too...
However, what is not clear to me, how would you handle s->has_write_zeroes
and s->has_discard in such a case? Set them to "false"? ... but it could
still work for some blocks with different alignment ... but if we keep them
set to "true", the code tries again and again to call these ioctls, maybe
wasting other precious cycles for this?
Maybe we should do a different approach instead: In case we hit a EINVAL
here, print an error a la:
error_report_once("You are running on a buggy file system, please complain
to the file system vendor");
and return -ENOTSUP ... then it's hopefully clear to the users why they are
getting a bad performance, and that they should complain to the file system
vendor instead to get their problem fixed.
Ping!
Any recommendations how to proceed here?
Thomas