Re: Software RAID1 (with non-identical discs) performance

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Wiebe Cazemier wrote:
When using non-identical discs (not just size, but also geometry) to contruct
your array, you can never get the partitions of the underlying discs to be
equal in size because the size of a partition can only be N*cylindersize,
where cylindersize varies across discs; the array always assumes the size of
the smallest partition. When one of the discs fails, you need to replace it
and make a partition that is exactly equal in size to the array, but because
that usually is impossible, it mostly will be bigger. To cover for this, I
have always left a small bit of unpartioned space on my discs. This not only
provides me with headroom in making the partitions on discs with different
geometry, but it's also possible that brand B's 250 GB is a little smaller
than brand A's, and staying (well) below the 250 GB, makes sure any 250 GB
disc fits in the array.

The entire concept of geometry is a a carryover from days gone by. These days it is just a farse maintained for backwards compatibility. You can put fdisk into sector mode with the 'u' command and create partitions of any number of sectors you desire, regardless of the perceived geometry.

My first question is, is this a necessary/convenient technique to ensure you
can replace discs over time, especially when you can't get the exact same
replacement disc?

I don't believe you need to do anything; md will simply not use the few extra sectors at the end of the larger disk/partition and round down to the appropriate size.

My second question is about the performance impact of using non-identical discs
and partitions. I can't really find any info about this, but I've read someone
making the statement that it would slow things down.

Yes, it slows things down. You want to try to match disk speeds as closely as possible for best performance.

My third question: write performance of RAID1 is usually lower than non-RAID,
because the data has to be sent over the bus twice. But, with for example an
NForce4 based mainboard using SATA, does that matter? I don't know if the SATA
ports are connected to the chipset by means of PCI express or hypertransport,
but both should be able to handle the double data transfer with room to spare.
So, as I understand it, as long as the kernel can perform both transfers
simultaniously, there should be no slow down, because when writing, there will
simply be two discs writing data simultaniously, at the same speed one drive
would. Is this correct?

Theoretically yes, more time will be spent sending the data across the bus twice, but most systems have enough spare capacity there that you probably won't notice.


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