[Ocfs2-users] Problem with OCFS2 disk on some moments (slow until stalls)
Tariq Saeed
tariq.x.saeed at oracle.com
Wed Sep 16 11:51:32 PDT 2015
On 09/16/2015 01:19 AM, Area de Sistemas wrote:
> Hi Tariq,
>
> DATA=WRITEBACK:
> From your words I understand that IF WE CAN ASSUME/TOLERATE A POSSIBLE
> FILES CONTENTS "CORRUPTION" IN CASE OF FAILURE, this option IMPROVES
> CLEARLY PERFORMANCE...right?
> * I ask to you this cause, according mount.ocfs2 man page, this option
> "is rumored to be the highest-throughput option"...which is a very
> vague/unclear idea of its benefits
I don't understand what exactly is unclear? Can you be more specific? Is
this a question or
a comment?
>
> COMMIT=XX (higher than 5s default):
> I am a bit confused: after searching on internet, many people
> recommends to use a value higher than 5s default (typically 30s or
> 60s) in case of performance issues, but you suggests that higher
> values can increment number of unecessary writes, that sounds the
> opposite so...can you clarify if/when commit="higher value than
> default" can be useful?
I stand corrected. I said the opposite of what I meant to say. Yes, the
higher the commit interval,
the longer the time lapse between syncing to disc, hence fewer the
comparative number of i/os due to
commit.
>
>
> THREADS BLOCKED ON MUTEX ON OCFS2 FILESYSTEM:
> From your words I understand that perhaps log area size is too small
> which can cause too "extra" I/Os in order to free/empty/clear log
> during high write loads...
> - There is some method to monitor the log usage?
I never had a need to do that and therefore don't know. If tunefs man
page does
not show anything, then there is none. You will have to google. ocfs2
uses jbd2, which is
used by other filesystems (ext3,4 ...) in Linux. The only control ocfs2
has over jbd2 is to call 'checkpoint', which means all blocks upto the
latest transaction need to be written
to their home locations on disc from the log area. This is i/o intensive
and happens
for example when a meta data cluster wide lock held in exclusive mode is
given up
(the on disk meta data protected by the lock must be upto date before
releasing it).
> * I've seen the tunefs.ocfs2 -J option and I suppose doesn't harm
> the filesystem but I prefer be sure about this. Anyway if I don't know
> the actual size of the log, I can't set an "acceptable" higher value
Again, I have not done it myself, so we are in the same
boat.tunfefs.ocfs2 should do it. You
should not lose data (unless there is a bug in tunefs.ocfs2). google is
your best source
for customer experiences.
>
> LOGS/ERRORS (ocfs2_unlink, ocfs2_rename, task blocked for more 120s):
> Apparently the load in the usage of the app continues being high BUT
> ALL THESE errors have dissapeared...
> * The usage pattern of the app these days is:
> - high number of users generating "backups" of partial contents of
> OCFS disk (so: high read+write) <--this is specific to the last weeks
> - "normal/low" reading access to contents
>
> CHANGES MADE AND ERRORS EVOLUTION:
> 1) First change:
> * two nodes disabled (httpd stopped but ocfs2 volume continues
> mounted) so ONLY ONE NODE IS SERVICING THE APP
> * After that, errors dissapeared...although %util was high
> 2) Three days after:
> * added commit=20 and data=writeback mount options to OCFS2 volume
> (maintaining only one node servicing app)
> * Situation persists: NO errors, although %util high
>
> So...It's possible that the concurrent use of the OCFS2 (2-3 nodes
> servicing app simultaneously) generate to much overload (caused by
> OCFS2 operation)?
You are on the spot. If you get high %util with one node, with more
nodes, there will be even
more apps served simultaneously, burning more disc bandwidth, which
seems to be the bottleneck here.
And then there is the overhead of internode communication through disk,
even if you don't service more
apps. Adding nodes gives you HA at cost of consuming some i/o bandwidth
and won't help you since
you already are using plenty of bandwidth.You should consider if you
already have not, stripping, making a volume out of many discs with the
logical volume manager etc.
Question: Are you using a separate disc for global heart beat?
> * Obviously, the OCFS2 operation generates an "extra" load
> but...perhaps under some circumstances (like these days usage of the
> app) the extra load becomes REALLY HIGH?
>
> Regards.
>
> ------------------------------------------------------------------------
>
> *Area de Sistemas
> Servicio de las Tecnologias de la Informacion y Comunicaciones (STIC)
> Universidad de Valladolid
> Edificio Alfonso VIII, C/Real de Burgos s/n. 47011, Valladolid - ESPAÑA
> Telefono: 983 18-6410, Fax: 983 423271
> E-mail: sistemas at uva.es
> *
>
> *
> ------------------------------------------------------------------------
> *
> El 16/09/15 a las 4:04, Tariq Saeed escribió:
>> Hi Area,
>> data=writeback improves things greatly. In ordered mode , the
>> default, before writing
>> a transaction(which only logs meta data changes) data is written.
>> This is very conservative
>> to ensure that before journal log buffer is written to disk journal
>> area, data has hit the disk and
>> transaction can be safely replayed in case of a crash -- only
>> complete transactions are replayed,
>> by complete I mean: begin-trans changebuf1, changebuf2, ... ,
>> changebufnn end-trans. Replay means buffer
>> are dispatched from the journal area on disk to their ultimate home
>> loc on disk. You can see now why
>> ordered mode generates so much i/o. In write back mode, transaction
>> can hit the disk but data
>> will be written whenever the kernel wants, asynchronously and without
>> knowing any relationship
>> to its related data. The danger is in case of a crash, we can replay
>> a transaction but its associated
>> data is not on disk. For example, if you truncate up a file to a new
>> bigger size and then
>> write something to a page beyond the old size, the page could hang
>> around in core for a long time
>> after transaction is written to the journal area on disk. If there is
>> a crash while the data page is still
>> in core, after replay, the file will have new size but the page with
>> data will show all zeros instead of
>> what you wrote. At any rate, this is a digression, just for your info.
>>
>> The commit is the interval at which data is synced to disc. I think
>> it may also be the interval
>> after which journal log buffer is written to disk. So decreasing it
>> reduces number of unecessary
>> writes.
>>
>> Now for the threads blocked for more than 120 sec in
>> /var/log/messages. There are two types.
>> First type is blocked on mutex on ocfs2 system file, mostly the
>> global bit map file shared by
>> all nodes. All writes to system files are done under transactions and
>> that may require
>> flushing to disk the journal buffer, depending upon your journal file
>> size. The smaller the size,
>> the fewer transactions it can hold, so more frequently the journal
>> log on disk needs to be
>> reclaimed by dispatching the meta data blocks from the journal space
>> to their home locations,
>> thus freeing up on-disk journal space. This requires reading meta
>> data blocks from journal area
>> on disk, and writing them to their home location. So again, lot of
>> i/o. I think the threads are
>> waiting on mutex because journal code must do this reclaiming to free
>> up space. The other kind
>> of blocked threads are NOT in ocfs2 code but they
>> all are blocked on mutex. I don't know why. That would require
>> getting a vmcore and chasing
>> the mutex owner and finding out why is it taking long time. I don't
>> think that is warranted at
>> this time.
>> Let me know if you have any further questions.
>> Thanks
>> -Tariq
>> On 09/15/2015 01:55 AM, Area de Sistemas wrote:
>>> Hi Tariq,
>>>
>>> Yesterday one node was under load but not as high as past week, and
>>> iostat showed:
>>> - 10% of samples with %util >90% (some peaks of 100%) and an average
>>> value of 18%
>>> - %iowait peaks of 37% with an average value of 4%
>>>
>>> BUT:
>>> - none of the indicated error messages appeared in /var/log/messages
>>> - we have mounted the OCFS2 filesystem with TWO extra options:
>>> data=writeback
>>> commit=20
>>> * Question about these extra options:
>>> Perhaps they help to mitigate in some way the problem?
>>> I've read about using them (usually commit=60) but I don't know
>>> if they really helps and/or they are even some other useful options
>>> to use
>>> Before, the volume as mounted using only the options
>>> "_netdev,rw,noatime"
>>>
>>> NOTE:
>>> - we have left only one node active (not the three nodes of the
>>> cluster) to "force" overloads
>>> - although only one node is serving the app, all the three nodes
>>> have the OCFS volume mounted
>>>
>>>
>>> About the EACCESS/ENOENT errors...we don't know if they are
>>> originated by:
>>> - an abnormal behavior of the application
>>> - the OCFS2 problem (a user tries to unlink/rename something and if
>>> system is slow due to OCFS the users retries again and again this
>>> operation, causing first operation to complete successfully but
>>> following fail)
>>> - a possible problem in the concurrency: now with only one node
>>> servicing the application errors doesn't appear but with the three
>>> nodes in service errors appeared (several nodes trying to do the
>>> same operation)
>>>
>>> And about the messages about blocked proccess in /var/log/messages
>>> I'll send directly to you (instead to the list) the file.
>>>
>>> Regards.
>>>
>>> ------------------------------------------------------------------------
>>>
>>> *Area de Sistemas
>>> Servicio de las Tecnologias de la Informacion y Comunicaciones (STIC)
>>> Universidad de Valladolid
>>> Edificio Alfonso VIII, C/Real de Burgos s/n. 47011, Valladolid - ESPAÑA
>>> Telefono: 983 18-6410, Fax: 983 423271
>>> E-mail: sistemas at uva.es
>>> *
>>>
>>> *
>>> ------------------------------------------------------------------------
>>> *
>>> El 14/09/15 a las 20:29, Tariq Saeed escribió:
>>>>
>>>> On 09/14/2015 01:20 AM, Area de Sistemas wrote:
>>>>> Hello everyone,
>>>>>
>>>>> We have a problem in a 3 member OCFS2 cluster used to serve an
>>>>> web/php application that access (read and/or write) files located
>>>>> in the OCFS2 volume.
>>>>> The problem appears only some times (apparently during high load
>>>>> periods).
>>>>>
>>>>> SYMPTOMS:
>>>>> - access to OCFS2 content becomes more an more slow until stalls
>>>>> * a "ls" command that normally takes <=1s takes 30s, 40s, 1m,...
>>>>> - load average of the system grows to 150, 200 or even more
>>>>>
>>>>> - high iowait values: 70-90%
>>>>>
>>>> This is hint that disk is under pressure. Run iostat (see
>>>> man page)
>>>> when this happens, producing report every 3 seconds or and
>>>> look at
>>>> %util col
>>>> %util
>>>> Percentage of CPU time during which I/O
>>>> requests were issued to the device (bandwidth
>>>> utilization for the device). Device saturation
>>>> occurs when this value is close to 100%.
>>>>
>>>>> * but CPU usage is low
>>>>>
>>>>> - in the syslog appears a lot of messages like:
>>>>> (httpd,XXXXX,Y):ocfs2_rename:1474 ERROR: status = -13
>>>> EACCES Permission denied. find the filename and check perms ls -l.
>>>>> or
>>>>> (httpd,XXXXX,Y):ocfs2_unlink:951 ERROR: status = -2
>>>> ENOENT All we can say is an attempt to delete a file from a
>>>> directory that has already been deleted.
>>>> This requires some knowledge of the
>>>> environment. Is there an application log.
>>>>>
>>>>> and the more "worrying":
>>>>> kernel: INFO: task httpd:3488 blocked for more than 120 seconds.
>>>>> kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
>>>>> disables this message.
>>>>> kernel: httpd D c6fe5d74 0 3488 1616 0x00000080
>>>>> kernel: c6fe5e04 00000082 00000000 c6fe5d74 c6fe5d74 000041fd
>>>>> c6fe5d88 c0439b18
>>>>> kernel: c0b976c0 c0b976c0 c0b976c0 c0b976c0 ed0f0ac0 c6fe5de8
>>>>> c0b976c0 f75ac6c0
>>>>> kernel: f2f0cd60 c0a95060 00000001 c6fe5dbc c0874b8d c6fe5de8
>>>>> f8fd9a86 00000001
>>>>> kernel: Call Trace:
>>>>> kernel: [<c0439b18>] ? default_spin_lock_flags+0x8/0x10
>>>>> kernel: [<c0874b8d>] ? _raw_spin_lock+0xd/0x10
>>>>> kernel: [<f8fd9a86>] ? ocfs2_dentry_revalidate+0xc6/0x2d0 [ocfs2]
>>>>> kernel: [<f8ff17be>] ? ocfs2_permission+0xfe/0x110 [ocfs2]
>>>>> kernel: [<f905b6f0>] ? ocfs2_acl_chmod+0xd0/0xd0 [ocfs2]
>>>>> kernel: [<c0873105>] schedule+0x35/0x50
>>>>> kernel: [<c0873b2e>] __mutex_lock_slowpath+0xbe/0x120
>>>>> ....
>>>>>
>>>> the important part of bt is cut off. Where is the rest of it? The
>>>> entries starting with "?"
>>>> are junk. You can attach /v/l/messages to give us a complete pic.My
>>>> guess is blocking on
>>>> mutex for so long is that the thread holding mutex is blocked on i/o.
>>>> Run "ps -e -o pid,stat,comm,whchan=WIDE_WCHAN-COLUMN" and look at
>>>> 'D' state (uninterruptable slee)
>>>> process. These are processes usually blocked on i/o.
>>>>>
>>>>> (UNACCEPTABLE) WORKAROUND:
>>>>> stop httpd (really slow)
>>>>> stop ocfs2 service (really slow)
>>>>> start ocfs2 an httpd
>>>>>
>>>>> MORE INFO:
>>>>> - OS information:
>>>>> Oracle Linux 6.4 32bit
>>>>> 4GB RAM
>>>>> uname -a: 2.6.39-400.109.6.el6uek.i686 #1 SMP Wed Aug 28
>>>>> 09:55:10 PDT 2013 i686 i686 i386 GNU/Linux
>>>>> * anyway: we have another 5 nodes cluster with Oracle Linux
>>>>> 7.1 (so 64bit OS) serving a newer version of the same application
>>>>> and the problems are similar, so it appears not to be a OS problem
>>>>> but a more specific OCFS2 problem (bug? some tuning? other?)
>>>>>
>>>>> - standard configuration
>>>>> * if you want I can show the cluster.conf configuration but is
>>>>> the "expected configuration"
>>>>>
>>>>> - standard configuration in o2cb:
>>>>> Driver for "configfs": Loaded
>>>>> Filesystem "configfs": Mounted
>>>>> Stack glue driver: Loaded
>>>>> Stack plugin "o2cb": Loaded
>>>>> Driver for "ocfs2_dlmfs": Loaded
>>>>> Filesystem "ocfs2_dlmfs": Mounted
>>>>> Checking O2CB cluster "MoodleOCFS2": Online
>>>>> Heartbeat dead threshold: 31
>>>>> Network idle timeout: 30000
>>>>> Network keepalive delay: 2000
>>>>> Network reconnect delay: 2000
>>>>> Heartbeat mode: Local
>>>>> Checking O2CB heartbeat: Active
>>>>>
>>>>> - mount options: _netdev,rw,noatime
>>>>> * so other options (commit, data, ...) have their default values
>>>>>
>>>>>
>>>>> Any ideas/suggestion?
>>>>>
>>>>> Regards.
>>>>>
>>>>> --
>>>>> ------------------------------------------------------------------------
>>>>>
>>>>> *Area de Sistemas
>>>>> Servicio de las Tecnologias de la Informacion y Comunicaciones (STIC)
>>>>> Universidad de Valladolid
>>>>> Edificio Alfonso VIII, C/Real de Burgos s/n. 47011, Valladolid -
>>>>> ESPAÑA
>>>>> Telefono: 983 18-6410, Fax: 983 423271
>>>>> E-mail: sistemas at uva.es
>>>>> *
>>>>>
>>>>> *
>>>>> ------------------------------------------------------------------------
>>>>> *
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Ocfs2-users mailing list
>>>>> Ocfs2-users at oss.oracle.com
>>>>> https://oss.oracle.com/mailman/listinfo/ocfs2-users
>>>>
>>>
>>
>
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