NevDull’s Take On Things

Why RAID 10 doesn’t help on EBS

Russell over at UKD1 did some benchmarking of Amazon’s new Elastic Block Store (I’m not loving the name, but the product certainly does look compelling), using a single device, and various numbers of drives in RAID 0 (striping), RAID 5 (striping with parity), and RAID 10 (striping across mirrors).

His results show a single drive maxing out at just under 65MB/s, RAID 0 hitting the ceiling at 110MB/s, RAID 5 maxxing out about 60MB/s, and RAID 10 “F2″ at under 55MB/s.

He then writes,

EBS. However, I would have expected RAID 10 F2 to have similar performance to RAID 0, rather than the results seen.

First of all, after glancing at this “F2″ variant of RAID 10, I’ve got to say that I haven’t bothered to fully grok it. I’ll give the benefit of the doubt that it’s performant. He shows the command-line he used, and it clearly has 3 component volumes in it, and even if I understood how one might use F2, 3 volumes in a RAID 10 seems clearly wrong.

But assuming that the system is intelligent enough to make sense of it…

I also haven’t ever mounted an EBS volume on the EC2 instance I’ve set up.

But one thing is obvious as to why the RAID 10 F2 volume performed poorly… it’s obviously bandwidth constrained, probably by a single gigabit ethernet connection. Software mirroring requires that data is written twice, once to each set of disks. When the disks are on separate controllers, they’re not competing on the same storage bus. Even when they’re on the same controller, but it’s SCSI or FibreChannel or SATA or whatever it may be — the bus has higher throughput than the disks do. You won’t saturate a U320 SCSI bus with a pair of disks, the disks are the bottleneck.

I’m quite impressed by the 110MB/s achieved through striping… now imagine that for every write, you’re clogging the pipe with 2 copies… what would you expect the capacity for each set of data to be? Strangely enough… 55MB/s.

QED

Amazon’s description of EBS refers to it as “highly available, highly reliable volumes”, which I take to mean fault tolerant – I’d assume RAID 1,5,6, or Z (Sun’s ZFS Raid) – which would make the mirroring portion of RAID 10 unnecessary because Amazon’s already taking care of the redundancy, so I don’t see this as a downside.

8 comments August 24th, 2008

GNU seq’s cousin on FreeBSD is… jot

This is another of my placeholders for reference.

I’ve got a FreeBSD system which is lacking some of the tools which I’ve gotten used to having, whether from Linux or Solaris.

I’ll often use the GNU tool seq to iterate through things on the command line… for example, if I’m going to ping 192.168.1.20-40, I might, at my bash prompt,

for i in `seq 20 40`; do ping 192.168.1.${i}; done

Quite handy, though FreeBSD doesn’t have it, and I haven’t installed whatever port contains it.

So… I’ll use jot, now that I’ve once again looked up what it is and how it works.

The equivalent line to that above?

for i in `jot 21 20`; do ping 192.168.1.${i}; done

6 comments September 24th, 2007

Sun Best Practices for Partitioning for Solaris Installation

Golly gee, in the few minutes since I decided to create a reference category, I had to find a reference to a Sun blueprints document I’ve referred to a few different times for work — including today. Time to make the blog a little more useful for everyday work, too…

The short version, in Sun’s words on page 3:

We recommend you follow this basic rule:
Do not partition unless there is a compelling reason to do so.

Here’s the Sun Blueprints document Configuring Boot Disks With Solaris™ Volume Manager Software

Add comment September 18th, 2007

Deduplicating lists in Excel

I’ve added a Reference category here on the ol’ blog to place links to things I may want to go back to.

Someone asked on IRC how to dedupe a list of things in Excel, and the process isn’t as intuitive as can be, so I thought I’d drop a quick post on here. Interestingly, there were a few commercial products trying to tout the same feature when I searched.

Here’s the Microsoft Office Online article entitled Delete duplicate rows from a list in Excel.

Add comment September 18th, 2007