Dailydave mailing list archives
Re: The Small Company's Guide to Hard Drive Failure and Linux
From: "Derek Vadala" <derek () cynicism com>
Date: Thu, 18 Nov 2004 15:34:03 -0500 (EST)
Anthony.zboralski said:
Make sure you stay away from "hardware RAID" as most of the implementations don't even support RAID5 and the performance is really poor 15 meg/second again 100+ with software raid. Plus you're stuck with a vendor with poor support.
Again, what you are calling hardware RAID here is probably firmware RAID. All the decent vendors can do RAID-5 in hardware. In good cases, this can be better than software RAID-5 due to caching. On-controller batteries also help with those pending writes when a power failure strikes. Software RAID-5 has good read speeds, like any RAID, but writes can be an issues. Imagine you have a five disk RAID-5 with a 5k stripe size (nevermind that this is not a valid stripe size). Each time you perform a write, you need to pre-read the parallel blocks in the stripe you are writing. This causes a lot of performance overhead for writes, and it's why a lot of people who can afford to use RAID-10 instead of RAID-5.
/dev/md0: Timing buffered disk reads: 168 MB in 3.01 seconds = 55.79 MB/sec root@dis:/home/acz# hdparm -T /dev/md0 /dev/md0: Timing cached reads: 1176 MB in 2.00 seconds = 586.62 MB/sec CPU usage is really minimal on this machine (1.8ghz AMD 2500+, 1gig of DDR ram),
Again, CPU overhead for RAID hasn't been a factor in a really long time. This is about I/O, not CPU-- just like nearly every computing bottleneck these days. The stats you posted are totally useless too. Even if they are accurate, which I doubt, they are for read performance. You're always going to get read performance that's near the speed of your real disks. I'm guessing your disks have a transfer rate of about 55MB/sec. Writes are what is important. Always. Reads are commodity.
RAID5 or RAID6 is really the best way to go in terms of security and performance. RAID5 allows 1 drive failure (and will rebuilt its state automatically if you have a spare) and RAID6 allows 2 drives to fail at the same time. Using other raid modes for anything is pure waste unless you work with big temporary files, for which the performance boost of a stripping array will come handy; 1 disk failure on a stripping array and you can say bye to your data.
Okay. This is one of those things that ends up buried in a mailing list archive for years, causing confusion among people trying to get good information. Most people don't want RAID-6. Most people don't care about surviving a multi-disk failure on a single array. Most people who need to do that will probably use a 3-disk RAID-1. That's right, you can create a RAID-1 of as many member disks as you like. Three disks means you get three copies of the data, four.... five... etc... RAID-5 and RAID-6 are not the best way to go in terms of security and performance. RAID-10 probably is, or perhaps even RAID-1. RAID-5 is great when you are poor and don't want to waste disk space, but there are drawbacks. RAID-6 inherits all of that and then some. _______________________________________________ Dailydave mailing list Dailydave () lists immunitysec com https://lists.immunitysec.com/mailman/listinfo/dailydave
Current thread:
- Re: Re: [nylug-talk] The Small Company's Guide to Hard Drive Failure and Linux, (continued)
- Re: Re: [nylug-talk] The Small Company's Guide to Hard Drive Failure and Linux Paul Wouters (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Paul Wouters (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Frank Berger (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Derek Vadala (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Dave Aitel (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux miah (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Anthony.zboralski (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Derek Vadala (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Anthony.zboralski (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Derek Vadala (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Anthony.zboralski (Nov 18)
- Re: The Small Company's Guide to Hard Drive Failure and Linux Derek Vadala (Nov 18)