There are a number of flaws in the blktap userspace daemon when dealing
with I/O errors.
- The backends which use AIO check the io_events.res member to determine
if an I/O error occurred. Which is good. But when calling the callback
to signal completion of the I/O, they pass the io_events.res2 member
Now this seems fine at first glance[1]
"res is the usual result of an I/O operation: the number of bytes
transfered, or a negative error code. res2 is a second status
value which will be returned to the user"
Except that
"currently (2.6.0-test9), callers of aio_complete() within the
kernel always set res2 to zero."
And this hasn't changed anytime since 2.6.0, so by passing through
the status from 'res2', the callback thinks the I/O operation succeeded
even when it failed :-(
The fix is simple instead of passing 'res2', just pass
ep->res == io->u.c.nbytes ? 0 : 1
This would solve the error reporting to the guest, except that there
is a second flaw...
- The tapdisk I/O completion callback checks the status parameter
passed in, syslog's it and then returns. It never bothers to send
the I/O completion response back to the blktap kernel driver when
a failure occurrs.
Fortunately the fix for this is also simple. Instead of returning
from the callback when dealing with an error, we simply toggle the
status field for the pending response to BLKIF_RSP_ERROR and then
continue with the normal codepath. So the error eventually gets
back to the guest.
The scenario I used to discover the problem and test the patch is thus:
- In dom0 create a filesystem with only 200 MB of free space
- Create a 1 GB sparse file on this volume.
- Configure the guest so this sparse file appears as /dev/xvdb
- In the domU create a single partition on /dev/xvdb and format
it with ext3.
- In the DomU, mount /dev/xvdb1 on /mnt and then run
dd if=/dev/zero of=/mnt/data.bin bs=1GB count=1
Without this patch, the 'dd' command would succeed in writing 1 GB of data
even though the underlying disk in Dom0 was only 200 MB in size. More complex
tests of copying a whole directory heirarchy across resulted in catastrophic
data corruption of the filessytem itself. Manual fsck was needed to fixup
the filesystem & there were many very bad errors needing fixing.
With this patch applied the DomU sees the I/O failures and kernel logs
messages
Dec 1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector
722127
Dec 1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector
730327
Dec 1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector
738527
Dec 1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector
746727
Dec 1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector
754927
Dec 1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector
763127
Dec 1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector
771327
Dec 1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector
779527
Dec 1 11:02:53 dhcp-5-203 kernel: end_request: I/O error, dev xvdc, sector
792399
It will retry the I/O operation until it runs out of sectors to try, and then
fail the operation. The filesystem is not seriously damaged - ext3 journal
recovery will trivially cleanup if the guest is rebooted after the disk in
Dom0 is enlarged.
Signed-off-by: Daniel P. Berrange <berrange@xxxxxxxxxx>
Regards,
Dan.
[1] http://lwn.net/Articles/24366/
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xen-blktap-report-errors.patch
Description: Text document
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