Re: EVMS performance/design question

Ross Boylan <[email protected]> wrote on 12/08/2006 12:27:19
PM:

> I've been moving mail to a Cyrus server on a new system, and it's
> speed seemed slow to me (I could only add c 20 messages/second).  I
> would like to run some tests with different options, but wanted to
> check here if they are meaningful.

> The system uses EVMS and LVM, and there's only one physical disk
> underneath it all.  My understanding is that both do lots of
> fsync'ing.

> If I set up separate regions from the LVM container, will an fsync on
> one of them only sync the region, or will it sync some larger object,
> such as the container or the physical disk?

> If I disable sync'ing on one of the logical volumes (via the
> filesystem or linux mount options--I've never done it, so I'm not sure
> how it goes), will that be effective?  (No, I don't think I'd disable
> it permanently, but I might for the migration.)

> I'm currently using ReiserFS v3, and a linux 2.6.17 kernel.

> Of course, any other suggestions about improving performance with my
> existing hardware (Intel P4 with hyperthreading and a SATA disk) would
> be great too!

> Thanks.
> Ross Boylan

Hi, Ross.

FWIW, here is my understanding on the topic.

EVMS and LVM themselves don't do any syncing.  They just provide block
devices.  It is the user (software or human) of the block devices that
decides if data should be flushed out of the page cache and onto the block
device.

fsync syncs the data for a particular file.  It should only force a write
of the file's data to the block device on which the filesystem resides.

sync flushes all page cache data for all filesystems to their underlying
block device.

The default option for a mount is async.  ReiserFS may not allow you mount
a filesystem sync.  From the mount man page: "The following options apply
to any file system that is being mounted (but not every file system
actually honors them - e.g., the sync option today has effect only for
ext2, ext3 and ufs)"  In general, Linux likes to do things asynchronously
since it performs better.  You usually have to ask for things to be done
synchronously.

There is no cache between the block device that implements a region and the
disk itself.  A write to the block device for the region causes a write to
the disk.

Do you have multiple volumes?  It could be that the system is hitting the
various filesystems but they are bottlenecking on the single disk.

Just some thoughts.  Hope it helps.

Steve D.


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