Red Hat Bugzilla – Bug 480663
data corruption and general brokenness with ramdisks (rd)
Last modified: 2010-10-23 03:07:43 EDT
Description of problem:
The ramdisk driver has long been known to have issues, particularly with a lot of page reclaim going on (memory pressure/swapping are good ways to help it misbehave). It was replaced upstream in 2.6.25 with a complete rewrite.
From Nick's description of the replacement driver:
"The old one is really difficult because it effectively implements a block
device which serves data out of its own buffer cache. It relies on the
dirty bit being set, to pin its backing store in cache, however there are
non trivial paths which can clear the dirty bit (eg.
try_to_free_buffers()), which had recently lead to data corruption. And
in general it is completely wrong for a block device driver to do this."
It's quite easy to trigger data corruption and other problems (hangs and oopses have been reported) by stressing the ramdisk driver or the VM.
Version-Release number of selected component (if applicable):
Takes a while.
Steps to Reproduce:
1. Create a number of large ramdisks
2. Configure the system with sufficiently little RAM that swapping will occur
3. Create and mount file systems on the ramdisks
4. Apply load to the ramdisk backed file systems (e.g. repeated tar/untar)
5. Verify data written to the devices at (4).
After some time (may take several days depending on I/O and memory load) data inconsistencies will appear. Normally manifests as a zero page in the middle of a ramdisk. Occasional hangs/oopses have been reported by end users but no data is currently available for these cases.
Reliable behavior of ramdisks under system load.
The user who reported this behavior has tested with the post-2.6.25 ramdisk rewrite and is unable to reproduce with those kernels.
New driver was merged in commit 9db5579be4bb5320c3248f6acf807aedf05ae143
Updating PM score.
You can download this test kernel from http://people.redhat.com/dzickus/el5
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I have downloaded kernel-2.6.18-157.el5.src.rpm to review based on some research I've done on the new brd (replacement ramdisk/rd) implementation. I am interested in it because our company makes heavy use of a ramdisk during certain data transformations and corruption would be bad(tm).
I've come across a few issues and do not see these patches having been applied in the rhel5.4 beta kernel release. In any event, a quick google of 'ext2 corruption brd', 'ext3 corruption brd', and 'ext4 corruption brd' will show 2 basic threads. They appear around Mar/Apr 2009.
In one, there appears to be probs with ext3 and ext4, which appears to be a prob with brd, ext3 jbd/jbd2, and htree's.
In the second, there appears to be probs with ext2.
Note: I'm not the author of any of these patches and not a kernel contributor... I've simply googled the info. Please don't apply these patches based on my authority without appropriate RHEL review ;)
Specific links and info below:
1. ext2 corruption (and proposed fix).
2. ext3 corruption in brd.c (and proposed fix).
3. ext3/4 corruption evidenced by jbd/jbd2 errors (and proposed fix)
Proposed fix: http://marc.info/?l=linux-ext4&m=123731584711382&w=2 (this link has 3 other fixes.. I have no idea what they are for).
Prob desc thread: http://osdir.com/ml/linux-ext4/2009-03/msg00274.html
4. ext3/4 corruption evidenced by corrupt htrees
I haven't found any evidence of a fix for this yet.
4a. The above link also refers to a patch titled "Nick's "fs: new inode
i_state corruption fix" patch".
Run the test script several days.
on i686, with kernel 2.6.18-92.el5 it was not able to reproduce the data corruption, and data corruption does not happen on 2.6.18-160.el5.
on x86_64, also data corruption does not happen.
The patch linux-2.6-misc-backport-new-ramdisk-driver.patch is include in kernel source package.
Hi Redhat, Hushan;
I took the opportunity after I posted to run the testing which uses fsstress. The command is in a while loop, and retrievable from one of the above links. I can post if need be. The actual fsstress command is this:
fsstress -d /mnt/test_file_system/work -p 3 -l 0 -n 100000000 -X &
Anyhow, having run the test, I can confirm corruption in ext2, ext3, and ext4 when running on a ramdisk (brd), and also a loop device (loop0), regardless of where I put that loop device (on a ramdisk, on a tmpfs /dev/shm, or in a file on an existing ext3 file system. The only time I have been successful is with ext3 mounted data=journal.
Furthermore, I took the opportunity to download the source code for kernel-2.6.18-157.el5 and linux-18.104.22.168 and can confirm the linux-2.6-misc-backport-new-ramdisk-driver.patch is missing code now upstream (again, the patch being a 1 liner).
There is an additional bug report in rhel bugzilla:
Hi Devin -
Thanks for the feedback.
The ext2/3 patches are not related to this bug. Please open a separate bz to follow those issues.
The brd.c patch is related and looks like a good addition to the existing brd patch.
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on therefore solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.