Slashdot Mirror

← Back to Stories (view on slashdot.org)

Are RAID Controllers the Next Data Center Bottleneck?

Posted by Soulskill on from the many-varied-pipes dept.

storagedude writes "This article suggests that most RAID controllers are completely unprepared for solid state drives and parallel file systems, all but guaranteeing another I/O bottleneck in data centers and another round of fixes and upgrades. What's more, some unnamed RAID vendors don't seem to even want to hear about the problem. Quoting: 'Common wisdom has held until now that I/O is random. This may have been true for many applications and file system allocation methodologies in the recent past, but with new file system allocation methods, pNFS and most importantly SSDs, the world as we know it is changing fast. RAID storage vendors who say that IOPS are all that matters for their controllers will be wrong within the next 18 months, if they aren't already.'"

2 of 171 comments (clear)

  1. BAD MATH by adisakp · · Score: 5, Interesting

    FTA Since a disk sector is 512 bytes, requests would translate to 26.9 MB/sec if 55,000 IOPS were done with this size. On the other end of testing for small block random is 8192 byte I/O requests, which are likely the largest request sizes that are considered small block I/O, which translates into 429.7 MB/sec with 55,000 requests

    I'm not going to believe an article that assumes that because you can do 55K IOPS for 512Byte reads, you can do the same number of IOPS for 8K reads which are 16X larger and then just extrapolate from there. Especially since most SSD's (at least SATA ones) right now top out around 200MB/s and the SATA interface tops out at 300MB/s. Besides there are already real world articles out there where guys with simple RAID0 SSD's are getting 500-600 MB with 3-4 drives using Motherboard RAID much less dedicated harware RAID.

  2. Re:I/O is random? What have you been smoking? by Anpheus · · Score: 5, Insightful

    All the important operations tend to be random. For a file server, you may have twenty people accessing files simultaneously. Or a hundred, or a thousand. For a webserver, it'll be hitting dozens or hundreds of static pages and, if you have database backend, that's almost entirely random as well.

    For people consolidating physical servers to virtual servers, you now have two, three, ten or twenty VMs running on one machine. If every one of those VMs tries to do a "sequential" IO, it gets interlaced by the hypervisor into all the other sequential IOs. No hypervisor would dare tell all the other VMs to sit back and wait so that every IO is sequential. That delay could be seconds or minutes or hours.

    Now imagine all that, and take into account that the latest Intel SSD gets around 6600 IOPS read and write. A good, fast hard drive gets 200. So you could put thirty three hard drives in RAID 0 and have the same number of IOPS, and your latency would still be worse. All the RAID0 really does for you is give you a nice big queue pipeline, like in a CPU. Your IO doesn't really get done faster, but you can have many more running simultaneously.

    Given that SSDs are easily three to four times faster on sequential IO and an order of magnitude faster on random IO, I don't think it's that implausible to believe that the industry isn't ready.