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Slow writes on disks

Tim Gordon
Advisor

Slow writes on disks

Hi,

I've got a performance issue with an RA4100 configured as one RAID 5 array of 11 x 36GB 15k disks with an online spare. The RA4100 is connected to a DL760 via a HUB7 and HBA. Checking the disk speed I am achieving reads from the disk a around 200 MB/s but writes are painfully slow at about 6 MB/s. If I copy a 1GB directory from one place on the disk to another it takes around 2.5 minutes. Is this correct?

I checked around some of our other servers and they all seem to be sufferring the same problem if indeed I have a problem! All systems are Windows 2003.

My question then, are these figures correct and does anyone know of a tool that I could use to check performance outside of the Windows OS?

Thanks in advance

Tim
7 REPLIES
Ivan Ferreira
Honored Contributor

Re: Slow writes on disks

I did a test copying 1GB from/to a ESA1200 RAID5 (14 disks 36 GB). The speed is 1 min 52 seg, so, i thingk that your write speed is more or less normal, considering that you are using windows, I use Tru64 Unix.
Por que hacerlo dificil si es posible hacerlo facil? - Why do it the hard way, when you can do it the easy way?
Tim Gordon
Advisor

Re: Slow writes on disks

Thanks for testing that. But shouldn't the speed of the disk writes be faster?

Tim
Uwe Zessin
Honored Contributor

Re: Slow writes on disks

A simple "COPY" is barely an appropriate measurement tool

Most likely, it reads the data from the disk, puts it in an internal memory buffer and then writes it to the destination disk - there is no double buffering. If you measure this, you are adding read and write times.

If you are copying within the same LUN, you are also creating additional head movements which results in a lower throughput as well.
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Tim Gordon
Advisor

Re: Slow writes on disks

I'm looking at this because SQL running on this box is slow to write. How can I improve write performance - but, perhaps more importantly, what write performance should I be getting?
Ruslan
Respected Contributor

Re: Slow writes on disks

Hi Tim!
The first rule of designing database storage is divide transaction and database files on different RAID partitions. If you want improve perfomance put t-logs on separate logigal drive with RAID 1+0 level. RAID 5 have writing overhead over RAID 1, therefor you system is being slowly
Tim Gordon
Advisor

Re: Slow writes on disks

Thanks,

The system drives are actually Raid 1 (the internal drives of the DL760). I have a total of 4 RA4100s (it is actually a pair of clustered DL760s). The logs are on one RA4100 and the data on another (both are actually RAID 5 as I needed to get the maximum capacity I could and so mirroring wasn't an option at the time.)

This particular hardware is not that relevant though as the performance is similar on other server kit we have - a write speed of around 6-8 MB per seconds. This to me seems slow.

Tim
John Kufrovich
Honored Contributor

Re: Slow writes on disks

Tim,
If your concern is SQL, then you should concentrate on IOPS performance, not MB/s. SQL does 8K blocks. I don't recall the MAX IOPS on the RA4100, but as your block transfer size rises, the IOPS will decrease and vice versa.