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Decreasing I/O latency in RAID array

Greg Schulz offers troubleshooting advice for a reader looking to decrease I/O latency in a RAID-5 array.

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We have around 10 users accessing our Linux-based RAID-5 SATA 2 TB storage server at a time. They import approximately 1.5 MB of more than 200 to 300 frames (TGA files) into their application and then render the same to the same storage. Our I/O latency is pretty high, sometimes their application barely responds and all the users connecting to this server feel the slowness. We would like to implement a better storage system and are looking at a SAN. Some dealers are pushing iSCSI, which we don't feel would suit our purpose, what would be your advice?
There are a lot of unknowns so it would be inappropriate to make a specific recommendation without more details of your environment. For example, I'm not clear if your clients are accessing 1.5 MByte-sized files, or if each user is doing 1.5Mbit/second I/O to the TGA files. Likewise, it's not clear if multiple users are accessing the same files or if they are working on unique files at any given time. How many SATA disk drives do you have configured in the RAID-5 setup, and do you have 2 TB usable storage or 2 TB total storage. For example, if you have eight 250 GB SATA disk drives (2 TB raw or total storage) that could work out to only 1.75 TB of RAID-5 storage (e.g. 7+1) or less if a hot spare disk drive were configured.

So, if you are doing lots of large writes and updates, you may want to look at not using RAID-5 given its parity overhead impact on performance. You might consider RAID-0+1 as well as look at migrating to higher performance SAS disk drives. While a NAS server may be a good fit, if you are using your Linux server for file serving that would accomplish the same purpose. However, take a look to see if you have adequate network bandwidth...

to your clients. iSCSI could be a good fit. However, if your users are accessing a shared volume, than NAS could be a good fit. On the other hand, iSCSI storage with SAS disk drives attached to your Linux server would be a good solution as well.

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How to minimize I/O latency?

How to implement SAS storage?

This was first published in April 2006

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