Re: RAID-1 to RAID-5 online migration?
G'day again,
From: "Andrew Miehs" <andrew@2sheds.de>
[...]
> Ummm... Bit confused here, but RAID 1 is not faster, than a single disk.
> RAID one is just for 'safety' purposes. Yes, you do have 2 disks, but
> in an
> ideal world, they will both be synced with one another, and both be
> doing
> exactly the same thing at the same time.
With RAID-1, both disks need to be written to at the same time, but for
reads you can read different data from each disk at once. This means, in
theory, that reading from RAID-1 can be 2x as fast.
However, whether you can actually read at 2x depends on how the read
requests are scheduled to the disks.
> > I suspect that reading 2 1G files at once on RAID-1 will be not much
> > faster than reading 2 1G files on a single disk, because reading two
> > files at once will probably be seek-bound, not throughput bound. RAID-1
> > boosts throughput, not latency.
I should have qualified this to; _can_ boost read-throughput... for the
right kinds of requests... if it's tuned right.
I was originally thinking that a single file read would read alternate data
blocks from alternate disks, and hence reading two files at once would cause
head-seeks on both disks between the two files.
Thinking about it more, for there to be any speed benefit, the length of
data read from each disk would have to be a whole track from each disk. A
whole track is kinda large, not giving you much "interleaving".
So Russell was right, reading two files at once is more likely to identify
any speed benefits than reading a single file. If the RAID-1 implementation
is smart enough, it can allocate read requests to different disks based on
"closest last read" to minimise seeks and allow simultaneous reads for
different read requests. Tuning this to get it right would be hard. I
wouldn't be surprised if most RAID-1 implementations don't bother.
----------------------------------------------------------------
Donovan Baarda http://minkirri.apana.org.au/~abo/
----------------------------------------------------------------
Reply to: