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Re: [Linux-cluster] gfs2 v. zfs?

>-----Original Message-----
>From: linux-cluster-bounces redhat com [mailto:linux-cluster-bounces redhat com] On Behalf Of Jeff Sturm
>Sent: Tuesday, 25 January 2011 09:02
>To: linux clustering
>Subject: Re: [Linux-cluster] gfs2 v. zfs?

>> -----Original Message-----
>> From: linux-cluster-bounces redhat com [mailto:linux-cluster-bounces redhat com]
>> On Behalf Of Wendy Cheng
>> Subject: Re: [Linux-cluster] gfs2 v. zfs?
>> Guess GFS2 is out as an "enterprise" file system ? W/out a workable backup solution,
>> it'll be seriously limited. I have been puzzled why CLVM is slow to add this feature.

>But an "enterprise" SAN will surely have native snapshots.  In fact, for production use we run GFS without CLVM.


A few comments, which might contrast uses of GFS2 and XFS in enterprise class production environments:

SAN snapshot is not a panacea, as it is only crash consistent and only within a single LUN.
If you have your data or database spread over multiple LUNs each with its own filesystem,
then you are on your own.

Therefore, we still need at least OS level (filesystem level) consistent backup
if the application itself does not provide a hot backup mechanism, which very few do.
The consistent filesystem level backup requires freeze and thaw commands.
XFS offers them, GFS2 does not.

GFS2 provides only tar(1) as a backup mechanism. 
Unfortunately, tar(1) does not cope efficiently with sparse files, 
which many applications create. 
As an exercise create a 10 TB sparse file with just one byte of non-null data at the end.
Then try to back it up to disk using tar(1). 
The tar image will be correctly created, but it will take many, many hours.
Dump(8) would do the job in a blink, but is not available for GFS2 filesystem.
However, XFS does have XFS specific dump(8) command and will backup sparse files

GFS2 is very convenient to use, as by its nature is clusterised.
However, there is huge performance cost to pay for all this convenience.
This cost stems from serialization imposed by distributed lock manager.

For these reason, for the HA applications running on one node at a time,
I found that XFS on top of LVM gives me the best mix of performance and functionality:
- high performance
- efficient backup of sparse files
- backup consistency through freeze/thaw
- zero downtime backup through use of LVM snapshots
- short failover times due to efficient XFS transaction logs

So, for this type of HA applications (failover HA) and environment, 
it makes perfect sense to use XFS in a cluster instead of GFS2.

Having said that, GFS2 can, in principle, be engineered to be much better
for failover HA applications.

It would require development of:
- GFS2 specific dump(8)
- GFS2 specific freeze and thaw commands
- CLVM wide snapshots
- more efficient DLM

It certainly is possible to do. Digital/Compaq/HP TruCluster Cluster File System (CFS) built on top of AdvFS had all of these features and much, much more by circa year 2000.


Chris Jankowski

Linux-cluster mailing list
Linux-cluster redhat com

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