Take these benchmarks with a grain of salt.

We don't know how these hardware controllers were setup and by the numbers posted, not very well, or they are not very good.

A SATA and a SAS drive will have roughly the same sequential io performance. Where SAS shines is in random io. So if it's archive, buy SATA.

65MB/s is roughly what you will see with a single SAS or SATA drive on reads, around 30MB/s for writes.

Sequential io is measured in MB/s and random io in IOPS or ios per second.

Each spindle in a stripe set will roughly add 50% perf to sequential io and add to the IOPS by the IOPS of the spindle (IOPS+IOPS...). A mirror counts as 1 spindle for reads and 1/2 a spindle for writes (unless RAID is capable of doing parallel reads then it counts as 1 1/2 of reads). A RAID 5 is always one less spindle due to parity and each spindle on writes counts as 1/#spindles (write-back cache helps lessen that hurt).

For 4k sequential ios (larger block sizes will post larger numbers).

1 spindle = 65MB/s and 175 IOPS 2 spindles = 97.5MB/s and 350 IOPS 3 spindles = 146.25MB/s and 525 IOPS 4 spindles = 219.375MB/s and 700 IOPS

(175 IOPS is from 15K SAS with 3.5ms read seek and 2ms avg latency, figure 80 IOPS for good SATA drive)

Now any performance below those numbers is a failure of the RAID system and any performance above those numbers is due to caching and read-ahead.

I hope that helps.

-Ross

----- Original Message ----- From: centos-bounces@centos.org centos-bounces@centos.org To: CentOS mailing list centos@centos.org Sent: Tue May 06 17:20:16 2008 Subject: Re: [CentOS] I need storage server advice

I just posted this on my website, oddly enough. While you need to really understand your storage requirements to make an informed choice between hardware or software RAID, with quad core CPUs being as cheap as they are it's hard to not make the argument for software. This is just hdparm over an average of 5 runs each on very similar machines.

5 disc SAS array with 136g 10k drives and a hardware controller

Timing cached reads: 13336 MB in 2.00 seconds = 6673.96 MB/sec Timing buffered disk reads: 98 MB in 1.18 seconds = 83.31 MB/sec

4 disc RAID 5 with 3Ware 9650SE and 500g 7200RPM drives

Timing cached reads: 6576 MB in 2.00 seconds = 3293.08 MB/sec Timing buffered disk reads: 448 MB in 3.00 seconds = 149.20 MB/sec

Single 500g 7200 RPM SATA drive

Timing cached reads: 14220 MB in 2.00 seconds = 7119.78 MB/sec Timing buffered disk reads: 198 MB in 3.02 seconds = 65.51 MB/sec

6 500g 7200 RPM SATA drives in a software RAID 5 array

Timing cached reads: 14364 MB in 2.00 seconds = 7191.86 MB/sec Timing buffered disk reads: 852 MB in 3.00 seconds = 283.64 MB/sec

Jason www.cyborgworkshop.org

Michael Semcheski wrote:

On Tue, May 6, 2008 at 3:11 PM, Ed Morrison <edward.morrison@gmail.com mailto:edward.morrison@gmail.com> wrote:

Situation:
My current storage needs are approximately 1.5 TB annually.  This
will increase to about 3.5 TB annually over the next 5 years (rough
est.).  This box will just be a data archive and once it is full it
will only be used very infrequently if not used at all. Files are
small up to 10 MB but numerous.

The solution I found best was to buy a 2U server that has 8*750GB disks, though they'd probably be 1TB today. Put the disks into a RAID 5 or 6. Using hardware RAID, divvy them up into one 50GB drive, and one really large drive. Put the OS on the 50GB drive, mount the really big drive.

Now you have a 50GB drive and a 7*750-50 drive. When you fill that up, just buy another 2U server. When you do fill it up, the next one will be cheaper and or bigger.

The keys to this type of setup are:

1. Don't buy storage you'll need next year today. The best time to buy

this kind of hardware is right before you need it. 2) Look at the overall cost per gigabyte. That's the metric that drives things. 3) Understand your tolerance for downtime and data protection. If you have another copy, or a backup, and its not mission critical data, its much cheaper not to waste disks on redundancy.

We have tape backups of our systems, and factoring in the cost of tape and other costs, its still possible to get storage with a marginal cost below $1 / GB. That includes a 3 year warranty, quad core processor, 4GB of RAM which you can probably put to use elsewhere.

---

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