> Will this bog down the system
No, not on any modern CPU. On an AMD Athlon II X2 245 (3 GHz), with
six 1.5TB disks in RAID6, it bounces beween 3%-7% CPU usage when doing a
5GB dd copy from /dev/zero to disk. But that's an I/O flood that
exceeds all caches, not standard usage. In everyday desktop use, you'll
never notice the software RAID. On my systems, I also add Linux's
full-disk encryption, which adds another 3%-8% CPU overhead. The power
of the CPU far outstrips the I/O capabilities of hard drive.
The one drawback is that you won't get the awesome parallelization
of I/O that a high-end RAID card would give you. I have a system here
with a low-end PCI SATA controller in it. For a 5GB test file on a
system with 4GB RAM, I see 160MB/s reads and 37MB/s writes. With full
disk encryption I see 105MB/s reads and 33MB/s writes. Those numbers
are about the same you got from a rack-mount hardware RAID server just
one or two years ago. That's fast enough for a SOHO Samba server, or a
video editing station, or a MythTV box. (It can flood a GigE network.)
To make the point about caches, I can copy a 200MB file and get
1.8GB/s read...
For bigger RAID filesystems, I have just now switched filesystems to
ext4. Some testing I read showed that ext4 can hold its own against
XFS. I shied away from it with the data loss issue last spring, but
that's been fixed and nothing new has shown up over the past couple of
years... I think it's time.
Note, if you're going to put a new OS on your RAID box, the Ubuntu
alternate install CD will let you create a RAID setup at install time.
There are walkthrough posted on the web someplace. You need the latest
(9.10) to get the RAID GUI with it.
--Derek
On 01/03/2010 10:13 PM, Ryan Allen wrote:
* Derek Simkowiak <[email protected]> wrote on [01-03-10y 15:35]:
May I ask, why must it be hardware RAID? Linux's software RAID is
usually as fast -- and sometimes faster -- than hardware RAID:
http://www.linux.com/news/hardware/servers/8222-benchmarking-hardware-raid-vs-linux-kernel-software-raid
I've built a handful of 4TB+ software RAID systems, and it works
great. Red Hat and Ubuntu (9.10) both include a nice RAID management
GUI called Palimpsest. Linux's software RAID also has a couple of
advanced features that most hardware cards don't. The best feature: you
can upgrade or migrate your hardware without any dependence on a
particular hardware RAID card. All you need is SATA.
I like this idea, after looking through the management interface of
palimpset. My only experience with software RAID was with a Terastation
1TB a few years back. It was so horribly slow, I got rid of it after 10
days. I don't remember the numbers, but it was borderline worthless.
How does a modern linux kernel running software RAID do with CPU usage?
A good example would be for transcoding video. Its very CPU intensive,
and fairly IO heavy, but in mostly contiguous blocks. Will this bog
down the system to the point that other users won't be able to make
light web requests, or incoming (fairly light) e-mail, will start to
drop due to excessive latency?
Of course all systems are different, I'd just like to know your
experiences.
Thanks,
-- Ryan
My advice would be to turn off the RAID function in your Dell card,
so it treats all the disk as simple SATA drives (sda, sdb, sdc ... sdf),
and then set up software RAID.
--Derek
P.S.> I recently lost data on a top-end hardware RAID system because a
3ware card had a compatibility issue with some hard drives. A few weeks
later both 3ware and the drive Mfr. issued firmware updates to "fix" it.
P.P.S.> I recently had a top-end LSI hardware RAID system fall over due
to an un-acknowledged drive compatibility issue. I had no choice but to
swap out the LSI card for a 3ware card. Did I mention that Linux
software RAID is awesome?
On 01/03/2010 03:11 PM, Ryan Allen wrote:
Hi SSL,
I just purchased 4 new 1TB SATA drives, and attempted to upgrade my
RAID 5 system to 3 TB. however my Dell CERC 6 channel SATA RAID card
will only let me build a RAID volume up to 2 TB.
I couldn't find any firmware upgrades on this horribly supported
el-cheepo raid card. At least its been a solid work horse, with
little problems aside from a horrible UI utility called afacli.
I am looking for a hardware RAID card, PCI-X (100MHz), that has an
fairly easy to use UI that is well supported in Linux. It should
also take advantage of ALL my drive space for an expected volume of a
little under 3TB.
Any suggestions? I would like to not spend $550 on an fancy 3ware
card either. Anything in the $100 range?
Thanks,
Ryan
