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Old 12-28-2009, 03:25 PM
Tom Bishop
 
Default Cnetos 5.4 ext3 question...

I could find this out but at the moment I dont have a copy of 5.4 running, can anyone tell me for a default centos5.4 install are ext3 barriers on by default? *Thanks in advance...*
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Old 12-28-2009, 03:48 PM
Ray Van Dolson
 
Default Cnetos 5.4 ext3 question...

On Mon, Dec 28, 2009 at 10:25:01AM -0600, Tom Bishop wrote:
> I could find this out but at the moment I dont have a copy of 5.4 running,
> can anyone tell me for a default centos5.4 install are ext3 barriers on by
> default? Thanks in advance...

Pretty sure they are off by default (and were on the box I just checked
against).

Ray
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Old 12-28-2009, 03:57 PM
Tom Bishop
 
Default Cnetos 5.4 ext3 question...

Thanks much, been reading about ext4 and performance issues, I've never had any issues with ext3 and my centos boxes...anyone else have any corruption issues when running ext3 as far as when power is cut etc...?


On Mon, Dec 28, 2009 at 10:48 AM, Ray Van Dolson <rayvd@bludgeon.org> wrote:

On Mon, Dec 28, 2009 at 10:25:01AM -0600, Tom Bishop wrote:

> I could find this out but at the moment I dont have a copy of 5.4 running,

> can anyone tell me for a default centos5.4 install are ext3 barriers on by

> default? *Thanks in advance...



Pretty sure they are off by default (and were on the box I just checked

against).



Ray

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Old 12-28-2009, 04:00 PM
Ray Van Dolson
 
Default Cnetos 5.4 ext3 question...

On Mon, Dec 28, 2009 at 10:57:10AM -0600, Tom Bishop wrote:
> Thanks much, been reading about ext4 and performance issues, I've never had
> any issues with ext3 and my centos boxes...anyone else have any corruption
> issues when running ext3 as far as when power is cut etc...?

Never had anything unrecoverable happen -- and have run servers in some
fairly harsh environments without decent UPS'es with a fair bit of IO
load on them as well..

Not to say you should't go for as much power redundancy as possible!

Ray
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Old 12-28-2009, 04:03 PM
 
Default Cnetos 5.4 ext3 question...

I'm using ext3 on my CentOS box, so far so good, I don't get any problem. Sometimes my server shutdown when power is cut, but CentOS still running well and nothing corruption files or anything after start again.

Ds.


-----Original Message-----
From: Tom Bishop <bishoptf@gmail.com>
Date: Mon, 28 Dec 2009 10:57:10
To: CentOS mailing list<centos@centos.org>
Subject: Re: [CentOS] Cnetos 5.4 ext3 question...

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Old 12-28-2009, 04:07 PM
Tom Bishop
 
Default Cnetos 5.4 ext3 question...

Thanks guys for the responses, can anyone explain what the hoopla is then about ext4 and performance issues and barriers being enabled, there was also some talk about that being an potential issue with ext3? *I've tried to google and look but have not found a good explanation on what the issue is....


On Mon, Dec 28, 2009 at 11:03 AM, <david@pnyet.web.id> wrote:

I'm using ext3 on my CentOS box, so far so good, I don't get any problem. Sometimes my server shutdown when power is cut, but CentOS still running well and nothing corruption files or anything after start again.




Ds.





-----Original Message-----

From: Tom Bishop <bishoptf@gmail.com>

Date: Mon, 28 Dec 2009 10:57:10

To: CentOS mailing list<centos@centos.org>

Subject: Re: [CentOS] Cnetos 5.4 ext3 question...



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Old 12-28-2009, 05:22 PM
Ross Walker
 
Default Cnetos 5.4 ext3 question...

On Dec 28, 2009, at 12:07 PM, Tom Bishop <bishoptf@gmail.com> wrote:


On Mon, Dec 28, 2009 at 11:03 AM, <david@pnyet.web.id> wrote:

I'm using ext3 on my CentOS box, so far so good, I don't get any problem. Sometimes my server shutdown when power is cut, but CentOS still running well and nothing corruption files or anything after start again.Thanks guys for the responses, can anyone explain what the hoopla is then about ext4 and performance issues and barriers being enabled, there was also some talk about that being an potential issue with ext3? *I've tried to google and look but have not found a good explanation on what the issue is....

Barriers expose the poor performance of cheap hard drives. They provide assurance that all the data leading up to the barrier and the barrier IO itself are committed to media. This means that the barrier does a disk flush first and if the drive supports FUA (forced unit access, ie bypass cache), then issues the IO request FUA, if the drive doesn't support FUA then it issues another cache flush. It's the double flush that causes the most impact to performance.
The typical fsync() call only assures data is flushed from memory, but makes no assurance the drive itself has flushed it to disk which is where the concern lies.
Currently in RHEL/CentOS the LVM (device mapper) layer doesn't know how to propogate barriers to the underlying devices so it filters them out, so barriers are only currently supported on whole drives or raw partitions. This is fixed in the current kernels, but has yet to be backported to RHEL kernels.
There are a couple of ways to avoid the barrier penalty. One is to have nvram backed write-cache either on the contoller or as a separate pass-through device. The other is to use a separate log device on a SSD which has nvram cache, newer ones have capacitor backed cache or a standalone nvram drive.
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Old 12-28-2009, 05:41 PM
Les Mikesell
 
Default Cnetos 5.4 ext3 question...

Ross Walker wrote:
> On Dec 28, 2009, at 12:07 PM, Tom Bishop <bishoptf@gmail.com
> <mailto:bishoptf@gmail.com>> wrote:
>
>>
>> On Mon, Dec 28, 2009 at 11:03 AM, <
>> <mailto:david@pnyet.web.id>david@pnyet.web.id
>> <mailto:david@pnyet.web.id>> wrote:
>>
>> I'm using ext3 on my CentOS box, so far so good, I don't get any
>> problem. Sometimes my server shutdown when power is cut, but
>> CentOS still running well and nothing corruption files or anything
>> after start again.
>> Thanks guys for the responses, can anyone explain what the hoopla is
>> then about ext4 and performance issues and barriers being enabled,
>> there was also some talk about that being an potential issue with
>> ext3? I've tried to google and look but have not found a good
>> explanation on what the issue is....
>
> Barriers expose the poor performance of cheap hard drives. They provide
> assurance that all the data leading up to the barrier and the barrier IO
> itself are committed to media. This means that the barrier does a disk
> flush first and if the drive supports FUA (forced unit access, ie bypass
> cache), then issues the IO request FUA, if the drive doesn't support FUA
> then it issues another cache flush. It's the double flush that causes
> the most impact to performance.
>
> The typical fsync() call only assures data is flushed from memory, but
> makes no assurance the drive itself has flushed it to disk which is
> where the concern lies.
>
> Currently in RHEL/CentOS the LVM (device mapper) layer doesn't know how
> to propogate barriers to the underlying devices so it filters them out,
> so barriers are only currently supported on whole drives or raw
> partitions. This is fixed in the current kernels, but has yet to be
> backported to RHEL kernels.
>
> There are a couple of ways to avoid the barrier penalty. One is to have
> nvram backed write-cache either on the contoller or as a separate
> pass-through device. The other is to use a separate log device on a SSD
> which has nvram cache, newer ones have capacitor backed cache or a
> standalone nvram drive.

Did linux ever get a working fsync() or does it still flush the entire
filesystem buffer?

--
Les Mikesell
lesmikesell@gmail.com
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Old 12-28-2009, 05:46 PM
Tom Bishop
 
Default Cnetos 5.4 ext3 question...

Thanks for the explanation, looks like I need to go read some more about barriers to truly understand what is going on.....

On Mon, Dec 28, 2009 at 12:22 PM, Ross Walker <rswwalker@gmail.com> wrote:

On Dec 28, 2009, at 12:07 PM, Tom Bishop <bishoptf@gmail.com> wrote:



On Mon, Dec 28, 2009 at 11:03 AM, <david@pnyet.web.id> wrote:


I'm using ext3 on my CentOS box, so far so good, I don't get any problem. Sometimes my server shutdown when power is cut, but CentOS still running well and nothing corruption files or anything after start again.
Thanks guys for the responses, can anyone explain what the hoopla is then about ext4 and performance issues and barriers being enabled, there was also some talk about that being an potential issue with ext3? *I've tried to google and look but have not found a good explanation on what the issue is....


Barriers expose the poor performance of cheap hard drives. They provide assurance that all the data leading up to the barrier and the barrier IO itself are committed to media. This means that the barrier does a disk flush first and if the drive supports FUA (forced unit access, ie bypass cache), then issues the IO request FUA, if the drive doesn't support FUA then it issues another cache flush. It's the double flush that causes the most impact to performance.

The typical fsync() call only assures data is flushed from memory, but makes no assurance the drive itself has flushed it to disk which is where the concern lies.

Currently in RHEL/CentOS the LVM (device mapper) layer doesn't know how to propogate barriers to the underlying devices so it filters them out, so barriers are only currently supported on whole drives or raw partitions. This is fixed in the current kernels, but has yet to be backported to RHEL kernels.

There are a couple of ways to avoid the barrier penalty. One is to have nvram backed write-cache either on the contoller or as a separate pass-through device. The other is to use a separate log device on a SSD which has nvram cache, newer ones have capacitor backed cache or a standalone nvram drive.

-Ross
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CentOS@centos.org

http://lists.centos.org/mailman/listinfo/centos




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Old 12-28-2009, 05:58 PM
Stephen Harris
 
Default Cnetos 5.4 ext3 question...

On Mon, Dec 28, 2009 at 12:46:24PM -0600, Tom Bishop wrote:
> Thanks for the explanation, looks like I need to go read some more about
> barriers to truly understand what is going on.....

(Please don't top post on these lists; thanks!)

As I understand it (but I could be wrong)... The problem is with "out
of order writes".

Typically with a journaled filesystem (like ext3) the system will write
out a datablock, then update the metadata (allocation tables, etc)
to reflect this. This order is important; the data must get to disk
before the metadata. Smart hardware, however, can optimise the writes so
it's possible for the metadata to get written to disk before the actual
datablocks; the result is potential data corruption (eg blocks allocated
with garbage in them) as opposed to potential data loss (eg blocks not
allocated) if the system dies with unwritten data in the buffer.

The work around for this is "barriers"; the system attempts to flush the
buffer to disk to ensure the datablock is written before the metadata.
Now blocks are written in the right order, but performance is lower
(flush flush).

"Barriers" are not currently implemented in the RHEL kernel for many
types of block device (including LVM devices).

--

rgds
Stephen
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