paul at clubi.ie wrote:
> On Fri, 16 Jan 2009, Kenn Humborg wrote:
>>> Interesting! How do Sun handle the boot-from-software-RAID
>> problem? They probably have much smarter boot loaders.
>> Sun wrote a ZFS module for GRUB. So GRUB knows enough to pick through
> ZFS, at least as in OpenSolaris.
>> There may be (probably are?) constraints on the RAID levels that can
> be used on a root-pool.
>> This code isn't in any supported version of Solaris, yet AFAIK (but
> might go into the next Solaris 10 update).
Most OS using software RAID can only boot from Mirror as without RAID
driver the "primary boot drive" looks like an ordinary drive. If the
Primary BIOS/SCSI/ATA/SATA drive doesn't work, you either physically
swap the two drives or change bios settings depending on OS, BIOS, type
of Interface etc.
So with SW RAID the safest approach is OS on a Mirror (RAID 1) and all
else on a single RAID5 or RAID6. Thus a minimum of 5 drives. (or 3 if
you partition all 3 and waste one primary partition, then part A of
Drive 0 and 1 is Mirror with OS and part B of Drive 0, 1, 2 is RAID 5.
However this is scary to fix when a drive fails).
Using HW RAID the CPU on the RAID controller "knows" how to boot RAID5
or RAID6 no matter which drive has failed. I used AMI Megaraid 428 for
RAID generally needs a UPS as a sudden power fail is VERY bad,
especially if SW RAID. Some HW RAID have battery backup on board and
store the status of the controller and cache during power fail.
Allegedly with UPS and autoshut down the battery is not needed.
Traditionally for various reasons SCSI was the best candidate for HW or
SW RAID. Certainly if you wanted a performance improvement (up to x100
possible on multiuser access 5 drives RAID 5 SCSI compared with a Single
IDE/SCSI) RAID5 on SCSI was needed. SW or HW. The problem was the
inability to seek on three ATA drives while writing on another and
reading the 5th etc.
Mirror speeds reads and slows write speed on SW RAID1 SCSI. It has
little impact on read speed and halves write time on PATA/IDE. RAID 5
can dramatically improve read and write performance as more drives
added, but added improvement drops as more drives added and risk of a
drive failure increases, so 4 or 5 drives is a sensible limit. I did
have a 7 drive RAID5 using 1G drives long ago. Never had two failures at
once or a failure during rebuild. it can happen but on good server class
SCSI drives very very much less likely than a drive fail, which is 7
times more likely than failure of a single drive system.
Some thought is needed for HW RAID configuration and MUCH thought is
needed for SW RAID configuration. I usually configured 2 swap files on
SW RAID, one on BOOT (mirror) and a faster one on RAID 5 data array.
Currently I have AMI megaraid 428 with 3 drive RAID 5 for OS and 5 drive
RAID 5 with bigger disks for Data. For performance these are all 10K
rpm Ultra Wide & Fast and the two RAID sets are on two
separate controller busses/ports. This doubles data rate and
reduces the issue of two drives at once problem if I had 8 same
size drives as one RAID set. The HW Controller is old (i960 based) and
does not support RAID6.
You can certainly max out the Gigibit ethernet even with RANDOM read or
Write, not block accesses. A single standalone Drive will drop maybe
over 100x to 500x in data performance due to latency and track to track
access time when used randomly by multiple users rather than single
sequential big file.
So RAID5 or RAID6 serves two purposes, Resilience on a failure (parity)
and dramatic improvement in random/multiuser performance due to stripe set
If you want simplicity, hot swap, and max performance, then HW RAID. If
you want cheaper and not so concerned about performance or ease of
administration SW RAID.
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