[WBEL-users] Software raid and User Mode Linux CD burning on
WBEL
Tim Moore
whitebox@nsr500.net
Tue, 03 Aug 2004 16:31:39 -0700
I've been using software RAID since the early patch days also and have
never had a problem. I've used a 3ware 6400 (4xATA/66) controller in both
RAID and non-RAID modes and have found no performance difference. As James
suggests after you get beyond ~450MHz celeron uniprocessor, the CPU
overhead is no longer important.
Note for mirror users: most if not all hardware controllers (3ware
included) scatter internal bits on the drives during initialization which
makes them NON-PORTABLE in case of a controller failure. I found this out
the hard trying to move half a 3ware RAID1 array to a different machine
after a motherboard failure.
Currently I run a modified RAID0 setup with rsync to a different physical
controller every two hours, and then to a different physical machine daily.
sda1 contains the boot block, / and /boot, and is dd'ed, including boot
block, to sdb1 as part of the primary rsync run. This allows a boot from
either physical disk without root raid issues simply by passing different
kernel params.
Performance is pretty much at the limits of ATA/66 on a matched pair of old
IBM IC35L020AVER07's (ATA drives show up as SCSI devices on the 3ware
controller):
[15:45] abit:~ > hdparm -tT /dev/sd{a,b}5 /dev/md0
/dev/sda5:
Timing buffer-cache reads: 656 MB in 2.00 seconds = 328.00 MB/sec
Timing buffered disk reads: 106 MB in 3.04 seconds = 34.87 MB/sec
/dev/sdb5:
Timing buffer-cache reads: 640 MB in 2.00 seconds = 320.00 MB/sec
Timing buffered disk reads: 106 MB in 3.01 seconds = 35.22 MB/sec
/dev/md0:
Timing buffer-cache reads: 652 MB in 2.01 seconds = 324.38 MB/sec
Timing buffered disk reads: 208 MB in 3.00 seconds = 69.33 MB/sec
[15:34] abit:~ > cat /proc/mdstat
Personalities : [raid0] [raid1] [raid5]
read_ahead 1024 sectors
md0 : active raid0 sdb5[1] sda5[0]
4192768 blocks 32k chunks
md1 : active raid0 sdb6[1] sda6[0]
2425600 blocks 32k chunks
md2 : active raid0 sdb7[1] sda7[0]
12707200 blocks 64k chunks
md3 : active raid0 sdb8[1] sda8[0]
20113152 blocks 64k chunks
unused devices: <none>
[15:34] abit:~ > fdisk -l /dev/sd{a,b}
Disk /dev/sda: 255 heads, 63 sectors, 2501 cylinders
Units = cylinders of 16065 * 512 bytes
Device Boot Start End Blocks Id System
/dev/sda1 * 1 13 104391 83 Linux
/dev/sda2 14 45 257040 82 Linux swap
/dev/sda3 46 2500 19719787+ 5 Extended
/dev/sda5 46 306 2096451 fd Linux raid autodetect
/dev/sda6 307 457 1212876 fd Linux raid autodetect
/dev/sda7 458 1248 6353676 fd Linux raid autodetect
/dev/sda8 1249 2500 10056658+ fd Linux raid autodetect
Disk /dev/sdb: 255 heads, 63 sectors, 2501 cylinders
Units = cylinders of 16065 * 512 bytes
Device Boot Start End Blocks Id System
/dev/sdb1 * 1 13 104391 83 Linux
/dev/sdb2 14 45 257040 82 Linux swap
/dev/sdb3 46 2500 19719787+ 5 Extended
/dev/sdb5 46 306 2096451 fd Linux raid autodetect
/dev/sdb6 307 457 1212876 fd Linux raid autodetect
/dev/sdb7 458 1248 6353676 fd Linux raid autodetect
/dev/sdb8 1249 2500 10056658+ fd Linux raid autodetect
[15:34] abit:~ > mount -l | grep data=
/dev/sda1 on / type ext3 (rw,data=ordered) [root]
/dev/md0 on /usr type ext3 (rw,noatime,data=writeback) [usr]
/dev/md1 on /home type ext3 (rw,noatime,data=writeback) [home]
/dev/md2 on /big type ext3 (rw,noatime,data=writeback) [big]
/dev/md3 on /spare type ext3 (rw,noatime,data=writeback) [spare]
/dev/hdc1 on /snapshot type ext3 (ro,data=journal) [snap]
James Knowles wrote:
>
>> Along these lines, does anyone know how much overhead Software RAID
>> introduces?none on /proc type proc (rw)
>
>
>
> I'd like to counter the idea that soft RAID introduces a lot of
> overhead. I've been using it since it was first introduced into the kernel.
>
> It doesn't really add much overhead, even on the pokey 233MHz server
> (soft RAID-5) that's been in service since 233MHz Pentrium-II was the
> rage of the day. For old machines I expect a 5% increase in CPU overhead
> with IDE, lower with SCSI. The worst I ever saw was 15% CPU increase
> under bitter load and IDE drives.
>
> Of course, on modern multi-GHz processors, soft RAID hardly impacts the
> machine.
>
> Since soft RAID was stabilized in the kernel years ago, I've never seen
> it put data at risk, including the harshest loads.
>
> Since hard drives have become cheap, we've always run our workstations
> with soft RAID-1, which has saved our bacon more than once over the
> years. It costs more to slap in an extra hard drive, but it's cheaper
> than what it does to a business on the rare day when a HD decides to
> roll over and die. The worst case is when the HD takes out swap, the
> kernel panics, and we have to yank the cable to the offending drive.
> When there's a failure, we can wait until after hours to actually swap
> out the drive.
>
> Anaconda has a nice interface for setting up soft RAID during install.
>
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