Posts Tagged ‘fs’

Xen Virtual Disk I/O Errors

I just encountered this while making a tar backup on one of my virtual filesystems:

lost page write due to I/O error on xvdb1
end_request: I/O error, dev xvdb1, sector 33707688
Buffer I/O error on device xvdb1, logical block 4213461

Unfortunately, my fs suffered corruption.

Some folks, particularly Ubuntu users with kernel 3.2.x seem to say the answer is adding

barrier=0

to your ext3/ext4 mount options.

In my case I’m pretty sure an OOM error killed something critical to the backend on dom0.

Be sure to check your dom0′s dmesg.

UPDATE Uhh yeah, something actually managed to overflow the dom0′s fs. >.> This is what I was getting in the dom0 dmesg:

loop: Write error at byte offset 17820364800, length 4096.

for which there is very little discussion, but a quick df told me what was going on.

Force File System Check (fsck) on Reboot

You may wish to check the integrity of your root file system without using a live cd or different system. To force a file system check on reboot either:

# touch /forcefsck

or

# shutdown -rF now

A Funny Thing Happened on the way to 100% Full tmpfs

I’ve been having trouble with XCache’s datastore lately and was prompted to revisit my SHM/tmpfs File-Based PHP Cache/Datastore in RAM strategy.

I mounted a 256MB tmpfs slice like this:

none                    /mnt/ram        tmpfs           defaults,noatime,size=256M      0 0

Somehow, after the storage was exhausted the number of files kept increasing.

$ watch "ls | wc -l"

To my surprise new, empty files were being created with a random string attached to their name.

36588.posts.object.ychanKJYGF
36588.posts.ychanPIUGN

Interestingly, while these non-files continued to accumulate the reported amount of storage used by tmpfs never changed. One might expect these file entries to be taking up some amount of space, somewhere.

I’ve done as much research as time will permit but haven’t found the purpose of this behaviour. Since the altered file name and lack of content makes these files void as far as my fcache implementation is concerned it highlights the importance of thorough garbage collection.

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