Brett Coady <bc196...@yahoo.com.au> writes:
[I *hope* I got the citations right here; for some reason your email didn't
include any of the normal bits in text you cited. :/ ]
> From: "Tony Sceats" <tony.sce...@gmail.com>
> To: slug@slug.org.au
>
>> Slower, though ... is a bit of a strange claim. Not because it is false,
>> but because the answer is complex: you can, for example, double read speed
>> and halve write speed, using a two disk RAID 1 array ... in the ideal case.
>
> I must say I'm curious about this, because I have always assumed that for a
> RAID 1 the write speed would be roughly the same as a single disk, not
> halved..
[...]
> I use a Kernel raid setup with 2 disks (Samsung 500GB), raid1 for /boot and
> raid0 for / and backup with dd to another drive every other week. This is
> just a desktop nothing too important, Raid 5 seems all the go from what I
> have read but I do not have the setup or time to look into it..
RAID5 has different performance trade-offs, and can have a (potentially
substantial) write penalty with the cheap-but-large consumer drives, under
random I/O. Watch out — getting them fast, rather than just big, is hard.
> My raid1 /boot
> /dev/md1:
> Timing cached reads: 7612 MB in 2.00 seconds = 3810.62 MB/sec
> Timing buffered disk reads: 244 MB in 3.01 seconds = 81.09 MB/sec
>
> 1 drive from the raid1 array above:
> /dev/sda1:
> Timing cached reads: 7490 MB in 2.00 seconds = 3749.14 MB/sec
> Timing buffered disk reads: 248 MB in 3.02 seconds = 82.01 MB/sec
The Linux RAID1 driver only balances reads between processes, not inside a
single process, so your average read performance for each application is going
to be the same as the slowest disk included.
The RAID10 driver, which can work with only two disks, does stripe reads for a
single process; if you want that, consider using that driver instead.
Daniel
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