Server Fault is a question and answer site for system and network administrators. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Scenario: we're running a large (3TB total in cluster) Solr installation in EC2. I'm only concerned about disk performance on the read side of things. We suspect that we are limited by disk I/O and specifically get problems when an EBS volume decides to get flaky and slow down.

I am considering trying out RAID 1. From what I understand Linux software RAID will split the reads between disks, but I'm not sure how smart it is. If one of the disks in a RAID 1 pair gets backed up on reads, will it send more reads to the other disk? If a read has been pending too long, will it reschedule that read to the other disk?

I can't think of any way to test this behavior. I'd be happy to look through the code, but I'm not even sure if this would be handled by the I/O scheduler or if it's part of the raid drivers. I can't find any documentation that goes into any more depth than the software RAID HowTo.

share|improve this question
Are there any writes? Lots of write traffic could block both disks. – geocar Jan 23 '10 at 0:44
FreeBSD's geom has somewhat tweakable software RAID1. – Fox Mar 29 '12 at 7:39
up vote 2 down vote accepted

The read balancing is handled in the md drivers in the kernel; drivers/md/raid1.c

It's not super clever. It tries to minimize seeks and that's about it. I'm not sure if any of the RAID implementations are as clever as you want...

Do you have enough memory in your systems? If it's really read-heavy, caching should help a bit.

share|improve this answer
I've gone through raid1.c, and I think the default behaviour is perfect for highly concurrent random reads. The only 'problem' is that it doesn't speed up sequential transfers by a single thread. – Seun Osewa May 5 '11 at 22:35

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.