On 13/09/16 21:00, Pranith Kumar Karampuri wrote:
On Tue, Sep 13, 2016 at 1:39 PM, Xavier Hernandez <xhernandez@xxxxxxxxxx
<mailto:xhernandez@xxxxxxxxxx>> 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.
I don't understand this case. You mean a write in an offset smaller than
the total size of the file that doesn't increment it ? if that's the
case, a sparse file could need to allocate new blocks even if the file
size doesn't change.
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?
We could do that, but this only represents a single case that later will
also be covered by the journal.
In any case, the solution here would be to restore previous file size
instead of (previous block - 1) * k, since this can cause the file size
to decrease. This works as long as we can assume that a failed write
doesn't touch previous contents.
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.
This is not possible with the current implementation, at least in an
efficient way. Only way to detect inconsistencies right now would be to
create all possible combinations of k bricks and compute the decoded
data for each of them. Then check if everything matches. If so, the
block is healthy, otherwise there is at least one damaged fragment. Then
it would be necessary to find a relation between the fragments used for
each reconstruction and the obtained data to determine a probable
candidate to be damaged.
Anyway, if there are 3 bad fragments in a 4+2 configuration, it won't be
possible to recover the data for that block. Of course, in this
particular case, this would mean that we would be able to recover all
file except the last written block.
With syndrome decoding (not currently implemented) and using a new
encoding matrix, this could be done in an efficient way.
Xavi
What do you guys think?
Xavi
Thanks and Regards,
Sanoj
----- Original Message -----
From: "Xavier Hernandez" <xhernandez@xxxxxxxxxx
<mailto:xhernandez@xxxxxxxxxx>>
To: "Raghavendra Gowdappa" <rgowdapp@xxxxxxxxxx
<mailto:rgowdapp@xxxxxxxxxx>>, "Sanoj Unnikrishnan"
<sunnikri@xxxxxxxxxx <mailto:sunnikri@xxxxxxxxxx>>
Cc: "Pranith Kumar Karampuri" <pkarampu@xxxxxxxxxx
<mailto:pkarampu@xxxxxxxxxx>>, "Ashish Pandey"
<aspandey@xxxxxxxxxx <mailto:aspandey@xxxxxxxxxx>>, "Gluster
Devel" <gluster-devel@xxxxxxxxxxx
<mailto:gluster-devel@xxxxxxxxxxx>>
Sent: Tuesday, September 13, 2016 11:50:27 AM
Subject: Re: Need help with
https://bugzilla.redhat.com/show_bug.cgi?id=1224180
<https://bugzilla.redhat.com/show_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@xxxxxxxxxx
<mailto:sunnikri@xxxxxxxxxx>>
To: "Pranith Kumar Karampuri" <pkarampu@xxxxxxxxxx
<mailto:pkarampu@xxxxxxxxxx>>, "Ashish Pandey"
<aspandey@xxxxxxxxxx <mailto:aspandey@xxxxxxxxxx>>,
xhernandez@xxxxxxxxxx <mailto:xhernandez@xxxxxxxxxx>,
"Raghavendra Gowdappa" <rgowdapp@xxxxxxxxxx
<mailto:rgowdapp@xxxxxxxxxx>>
Sent: Monday, September 12, 2016 7:06:59 PM
Subject: Need help with
https://bugzilla.redhat.com/show_bug.cgi?id=1224180
<https://bugzilla.redhat.com/show_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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