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Re: mail queue's, ext3 data=journal and sync-mount

On Tue, 2002-08-20 00:42:31 +0200, Russell Coker <russell@coker.com.au>
wrote in message <[🔎] 20020819224232.4294A129F@lyta.coker.com.au>:
> On Mon, 19 Aug 2002 17:17, you wrote:
> > True.  Do you know why ext2 sync-mounted is so abysmally slow?  I mean,
> > our RAID was barely breaking a sweat, and bonnie++ was barely using 2-3%
> > CPU, and yet, things just wouldn't go any faster, what's the bottleneck?
> Write back caching is simply a great way of improving performance.
> If you have a single hard drive then when writing to a file, even if the data 
> is all contiguous (the file is not fragmented) then when writing data for 
> each write the disk will need to spin to the correct location before data can 
> be written, for a 10K rpm drive that'll be an average of 3ms overhead per 
> chunk.  Use a larger chunk size for Bonnie++ and performance should improve.
> Also for a RAID-5 it's even worse.  To write to a sector on a RAID-5 you have 
> to do two reads and two writes minimum (or a read from all disks minus two 
> plus two writes) to get the correct parity.  For a three disk RAID-5 that's 
> one read and two writes, for a five disk RAID-5 it's two reads and two writes.
> If you write the entire stripe at once (could be dozens of blocks depending 
> on the RAID setup) then it's little overhead when compared to a non-RAID 
> setup (RAID-5 should perform well for writing big files non-synchronously).
> Again make the chunk size larger on Bonnie++ and you should see a good 
> performance improvement.
> You might even discover that the performance of your RAID setup can be 
> measured in synchronous writes per second rather than any other metric.

within this discussion, i got the idea to put an external journal for
the ext3fs on an raid1-volume the real data on a raid5, hopefully, when
writing the journal out to the disk having more data to be written an
once - this would be worth a try but i don't have the hardware to try
this and bad expirences with software raid5.


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