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тАО09-09-2005 06:03 AM
тАО09-09-2005 06:03 AM
Slow writes on disks
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
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тАО09-09-2005 07:26 AM
тАО09-09-2005 07:26 AM
Re: Slow writes on disks
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тАО09-11-2005 07:50 PM
тАО09-11-2005 07:50 PM
Re: Slow writes on disks
Tim
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тАО09-11-2005 08:08 PM
тАО09-11-2005 08:08 PM
Re: Slow writes on disks
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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тАО09-11-2005 08:13 PM
тАО09-11-2005 08:13 PM
Re: Slow writes on disks
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тАО09-11-2005 09:50 PM
тАО09-11-2005 09:50 PM
Re: Slow writes on disks
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
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тАО09-11-2005 09:54 PM
тАО09-11-2005 09:54 PM
Re: Slow writes on disks
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
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тАО09-12-2005 01:51 AM
тАО09-12-2005 01:51 AM
Re: Slow writes on disks
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.