[Gluster-devel] Need help with https://bugzilla.redhat.com/show_bug.cgi?id=1224180

Tue Sep 13 19:00:52 UTC 2016

On Tue, Sep 13, 2016 at 1:39 PM, Xavier Hernandez <xhernandez at datalab.es>
wrote:

> Hi Sanoj,
>
> On 13/09/16 09:41, Sanoj Unnikrishnan wrote:
>
>> Hi Xavi,
>>
>> That explains a lot,
>> I see a couple of other scenario which can lead to similar inconsistency.
>> 1) simultaneous node/brick crash of 3 bricks.
>>
>
> Although this is a real problem, the 3 bricks should crash exactly at the
> same moment just after having successfully locked the inode being modified
> and queried some information, but before sending the write fop nor any down
> notification. The probability to have this suffer this problem is really
> small.
>
> 2) if the disk space of underlying filesystem on which brick is hosted
>> exceeds for 3 bricks.
>>
>
> Yes. This is the same cause that makes quota fail.
>
>
>> I don't think we can address all the scenario unless we have a
>> log/journal mechanism like raid-5.
>>
>
> I completely agree. I don't see any solution valid for all cases. BTW
> RAID-5 *is not* a solution. It doesn't have any log/journal. Maybe
> something based on fdl xlator would work.
>
> Should we look at a quota specific fix or let it get fixed whenever we
>> introduce a log?
>>
>
> Not sure how to fix this in a way that doesn't seem too hacky.
>
> One possibility is to request permission to write some data before
> actually writing it (specifying offset and size). And then be sure that the
> write will succeed if all (or at least the minimum number of data bricks)
> has acknowledged the previous write permission request.
>
> Another approach would be to queue writes in a server side xlator until a
> commit message is received, but sending back an answer saying if there's
> enough space to do the write (this is, in some way, a very primitive
> log/journal approach).
>
> However both approaches will have a big performance impact if they cannot
> be executed in background.
>
> Maybe it would be worth investing in fdl instead of trying to find a
> custom solution to this.
>

There are some things we shall do irrespective of this change:
1) When the file is in a state that all 512 bytes of the fragment represent
the data, then we shouldn't increase the file size at all which discards
the write without any problems, i.e. this case is recoverable.
2) when we append data to a partially filled chunk and it fails on 3/6
bricks, the rest could be recovered by adjusting the file size to the size
represented by (previous block - 1)*k, we should probably provide an option
to do so?
3) Proivde some utility/setfattr to perform recovery based on data rather
than versions. i.e. it needs to detect and tell which part of data is not
recoverable and which can be. Based on that, the user should be able to
recover.

What do you guys think?

> Xavi
>
>
>
>> Thanks and Regards,
>> Sanoj
>>
>> ----- Original Message -----
>> From: "Xavier Hernandez" <xhernandez at datalab.es>
>> To: "Raghavendra Gowdappa" <rgowdapp at redhat.com>, "Sanoj Unnikrishnan" <
>> sunnikri at redhat.com>
>> Cc: "Pranith Kumar Karampuri" <pkarampu at redhat.com>, "Ashish Pandey" <
>> aspandey at redhat.com>, "Gluster Devel" <gluster-devel at gluster.org>
>> Sent: Tuesday, September 13, 2016 11:50:27 AM
>> Subject: Re: Need help with https://bugzilla.redhat.com/sh
>> ow_bug.cgi?id=1224180
>>
>> Hi Sanoj,
>>
>> I'm unable to see bug 1224180. Access is restricted.
>>
>> Not sure what is the problem exactly, but I see that quota is involved.
>> Currently disperse doesn't play well with quota when the limit is near.
>>
>> The reason is that not all bricks fail at the same time with EDQUOT due
>> to small differences is computed space. This causes a valid write to
>> succeed on some bricks and fail on others. If it fails simultaneously on
>> more than redundancy bricks but less that the number of data bricks,
>> there's no way to rollback the changes on the bricks that have
>> succeeded, so the operation is inconsistent and an I/O error is returned.
>>
>> For example, on a 6:2 configuration (4 data bricks and 2 redundancy), if
>> 3 bricks succeed and 3 fail, there are not enough bricks with the
>> updated version, but there aren't enough bricks with the old version
>> either.
>>
>> If you force 2 bricks to be down, the problem can appear more frequently
>> as only a single failure causes this problem.
>>
>> Xavi
>>
>> On 13/09/16 06:09, Raghavendra Gowdappa wrote:
>>
>>> +gluster-devel
>>>
>>> ----- Original Message -----
>>>
>>>> From: "Sanoj Unnikrishnan" <sunnikri at redhat.com>
>>>> To: "Pranith Kumar Karampuri" <pkarampu at redhat.com>, "Ashish Pandey" <
>>>> aspandey at redhat.com>, xhernandez at datalab.es,
>>>> "Raghavendra Gowdappa" <rgowdapp at redhat.com>
>>>> Sent: Monday, September 12, 2016 7:06:59 PM
>>>> Subject: Need help with https://bugzilla.redhat.com/sh
>>>> ow_bug.cgi?id=1224180
>>>>
>>>> Hello Xavi/Pranith,
>>>>
>>>> I have been able to reproduce the BZ with the following steps:
>>>>
>>>> gluster volume create v_disp disperse 6 redundancy 2
>>>> $tm1:/export/sdb/br1
>>>> $tm2:/export/sdb/b2 $tm3:/export/sdb/br3  $tm1:/export/sdb/b4
>>>> $tm2:/export/sdb/b5 $tm3:/export/sdb/b6 force
>>>> #(Used only 3 nodes, should not matter here)
>>>> gluster volume start v_disp
>>>> mount -t glusterfs $tm1:v_disp /gluster_vols/v_disp
>>>> mkdir /gluster_vols/v_disp/dir1
>>>> dd if=/dev/zero of=/gluster_vols/v_disp/dir1/x bs=10k count=90000 &
>>>> gluster v quota v_disp enable
>>>> gluster v quota v_disp limit-usage /dir1 200MB
>>>> gluster v quota v_disp soft-timeout 0
>>>> gluster v quota v_disp hard-timeout 0
>>>> #optional remove 2 bricks (reproduces more often with this)
>>>> #pgrep glusterfsd | xargs kill -9
>>>>
>>>> IO error on stdout when Quota exceeds, followed by Disk Quota exceeded.
>>>>
>>>> Also note the issue is seen when A flush happens simultaneous with quota
>>>> limit hit, Hence Its not seen only on some runs.
>>>>
>>>> The following are the error in logs.
>>>> [2016-09-12 10:40:02.431568] E [MSGID: 122034]
>>>> [ec-common.c:488:ec_child_select] 0-v_disp-disperse-0: Insufficient
>>>> available childs for this request (have 0, need 4)
>>>> [2016-09-12 10:40:02.431627] E [MSGID: 122037]
>>>> [ec-common.c:1830:ec_update_size_version_done] 0-Disperse: sku-debug:
>>>> pre-version=0/0, size=0post-version=1865/1865, size=209571840
>>>> [2016-09-12 10:40:02.431637] E [MSGID: 122037]
>>>> [ec-common.c:1835:ec_update_size_version_done] 0-v_disp-disperse-0:
>>>> Failed
>>>> to update version and size [Input/output error]
>>>> [2016-09-12 10:40:02.431664] E [MSGID: 122034]
>>>> [ec-common.c:417:ec_child_select] 0-v_disp-disperse-0: sku-debug:
>>>> mask: 36,
>>>> ec->xl_up 36, ec->node_mask 3f, parent->mask:36, fop->parent->healing:0,
>>>> id:29
>>>>
>>>> [2016-09-12 10:40:02.431673] E [MSGID: 122034]
>>>> [ec-common.c:480:ec_child_select] 0-v_disp-disperse-0: sku-debug:
>>>> mask: 36,
>>>> remaining: 36, healing: 0, ec->xl_up 36, ec->node_mask 3f,
>>>> parent->mask:36,
>>>> num:4, minimum: 1, id:29
>>>>
>>>> ...
>>>> [2016-09-12 10:40:02.487302] W [fuse-bridge.c:2311:fuse_writev_cbk]
>>>> 0-glusterfs-fuse: 41159: WRITE => -1
>>>> gfid=ee0b4aa1-1f44-486a-883c-acddc13ee318 fd=0x7f1d9c003edc
>>>> (Input/output
>>>> error)
>>>> [2016-09-12 10:40:02.500151] W [MSGID: 122006]
>>>> [ec-combine.c:206:ec_iatt_combine] 0-v_disp-disperse-0: Failed to
>>>> combine
>>>> iatt (inode: 9816911356190712600-9816911356190712600, links: 1-1, uid:
>>>> 0-0,
>>>> gid: 0-0, rdev: 0-0, size: 52423680-52413440, mode: 100644-100644)
>>>> [2016-09-12 10:40:02.500188] N [MSGID: 122029]
>>>> [ec-combine.c:93:ec_combine_write] 0-v_disp-disperse-0: Mismatching
>>>> iatt in
>>>> answers of 'WRITE'
>>>> [2016-09-12 10:40:02.504551] W [MSGID: 122006]
>>>> [ec-combine.c:206:ec_iatt_combine] 0-v_disp-disperse-0: Failed to
>>>> combine
>>>> iatt (inode: 9816911356190712600-9816911356190712600, links: 1-1, uid:
>>>> 0-0,
>>>> gid: 0-0, rdev: 0-0, size: 52423680-52413440, mode: 100644-100644)
>>>> ....
>>>> ....
>>>>
>>>> [2016-09-12 10:40:02.571272] N [MSGID: 122029]
>>>> [ec-combine.c:93:ec_combine_write] 0-v_disp-disperse-0: Mismatching
>>>> iatt in
>>>> answers of 'WRITE'
>>>> [2016-09-12 10:40:02.571510] W [MSGID: 122006]
>>>> [ec-combine.c:206:ec_iatt_combine] 0-v_disp-disperse-0: Failed to
>>>> combine
>>>> iatt (inode: 9816911356190712600-9816911356190712600, links: 1-1, uid:
>>>> 0-0,
>>>> gid: 0-0, rdev: 0-0, size: 52423680-52413440, mode: 100644-100644)
>>>> [2016-09-12 10:40:02.571544] N [MSGID: 122029]
>>>> [ec-combine.c:93:ec_combine_write] 0-v_disp-disperse-0: Mismatching
>>>> iatt in
>>>> answers of 'WRITE'
>>>> [2016-09-12 10:40:02.571772] W [fuse-bridge.c:1290:fuse_err_cbk]
>>>> 0-glusterfs-fuse: 41160: FLUSH() ERR => -1 (Input/output error)
>>>>
>>>> Also, for some fops before the write I noticed the fop->mask field as
>>>> 0, Its
>>>> not clear why this happens ??
>>>>
>>>> [2016-09-12 10:40:02.431561] E [MSGID: 122034]
>>>> [ec-common.c:480:ec_child_select] 0-v_disp-disperse-0: sku-debug:
>>>> mask: 0,
>>>> remaining: 0, healing: 0, ec->xl_up 36, ec->node_mask 3f,
>>>> parent->mask:36,
>>>> num:0, minimum: 4, fop->id:34
>>>> [2016-09-12 10:40:02.431568] E [MSGID: 122034]
>>>> [ec-common.c:488:ec_child_select] 0-v_disp-disperse-0: Insufficient
>>>> available childs for this request (have 0, need 4)
>>>> [2016-09-12 10:40:02.431637] E [MSGID: 122037]
>>>> [ec-common.c:1835:ec_update_size_version_done] 0-v_disp-disperse-0:
>>>> Failed
>>>> to update version and size [Input/output error]
>>>>
>>>> Is the zero value of fop->mask related to mismatch in iatt ?
>>>> Any scenario of race between write/flush fop?
>>>> please suggest how to proceed.
>>>>
>>>> Thanks and Regards,
>>>> Sanoj
>>>>
>>>>

-- 
Pranith
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