[Users] ploop on ext4 without journal - bug?

[Dmitry Monakhov]

Sergey Bronnikov <sergeyb at openvz.org> writes:

> Dima, could you help?
>
> On 02:08 Wed 30 Sep , Nick Knutov wrote:
>> Hello all,
>> 
>> I have an ext4 partition without journal (I need it so):
First of all. The subject you mentioned is incorrect. This is not
nojournal mode. Configuration you want to create is external journal with data=journal.

data=journal is full data journaling mode. Such mode assumes that it
will pass through journal all data, but ploop directly issues bios to
lower-fs(i.e. baypass journal). This done for performance reasons. That
is why ploop is faster that any other solutions.
All this means that full journaling for lower(/vz/private) fs is not
compatible with ploop. So please do not use it, otherwise you'll get
undefined behavior (most likely silent corruptions in guest-fs)

The glitch you have mentioned most likely happen due to the fact that
you use SSD. Recently we have found a bug in mm reclaim code which
result in deadlock (swap on ssd in our case) https://jira.sw.ru/browse/PSBM-39335

Bug was fixed here:
*diff-ms-mm-vmscan-do-not-wait-for-page-writeback-for-GFP_NOFS-allocations
Added to 042stab112_3

mm, vmscan: Do not wait for page writeback for GFP_NOFS
Backport of mainline patch ecf5fc6e9654

>> 
>> mount | grep vz2
>> /dev/sde1 on /vz2 type ext4 (rw,relatime,discard,errors=remount-ro,commit=20,data=journal,journal_async_commit)
>> 
>> debugfs -R features /dev/sde1
>> debugfs 1.41.12 (17-May-2010)
>> Filesystem features: ext_attr resize_inode dir_index filetype extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
>> 
>> When I'm trying to create CT with ploop layout - I've got
>> 
>> Creating image: /vz2/private/2008.tmp/root.hdd/root.hdd size=10485760K
>> Creating delta /vz2/private/2008.tmp/root.hdd/root.hdd bs=2048 size=20971520 sectors v2
>> Storing /vz2/private/2008.tmp/root.hdd/DiskDescriptor.xml
>> WARNING: /vz2 is mounted with data=writeback not recommended for ploop; please use data=ordered instead
>> Opening delta /vz2/private/2008.tmp/root.hdd/root.hdd
>> Adding delta dev=/dev/ploop58376 img=/vz2/private/2008.tmp/root.hdd/root.hdd (rw)
>> 
>> and now it freezes. (btw, vzctl says it's data=writeback, but it's
>> data=journal and journal is removed - is it ok?)
>> 
>> 
>> When ctrl+c I've got:
>> 
>> ^C
>> Cancelling...
>> Cancelling...
>> Destroying container private area: /vz2/private/2008.tmp
>> ^C
>> Cancelling...
>> Cancelling...
>> 
>> so I have to log in other ssh session and kill -9 it.
>> 
>> Kernel: 042stab108.8
>> 
>> Is it a bug or I'm doing something wrong?
>> 
>> -- 
>> Best Regards,
>> Nick Knutov
>> http://knutov.com
>> ICQ: 272873706
>> Voice: +7-904-84-23-130 
>> 
>
>> _______________________________________________
>> Users mailing list
>> Users at openvz.org
>> https://lists.openvz.org/mailman/listinfo/users
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