Re: md-raid and block sizes

["D.S. Ljungmark" <spider@skuggor.se>]

Thankyou, 
  This makes planning ahead a bit easier, and means that I "only" have
to worry about the traditional issues regarding block sizes, stripe and
stride on "naked" disks in the array. 

Regards,
   Spid

On Mon, 2010-12-27 at 10:59 -0800, Doug Dumitru wrote:
> Mr. Ljungmark,
> 
> The meta data is stored at the end of each raid device.  So if you are
> dealing with an "advanced partitioned" device, just make sure your
> individual elements are stores with alignments that make sense.  Normally,
> this is just 'fdisk -u' and start the partition on a multiple of 8 sectors
> (start at 64).
> 
> In terms of stripe sizes, they are all multiples of 4K pages anyway, so it
> is not really possible for them to be "wrong" in terms of drive format
> alignment.
> 
> File systems should be 4K or multiples thereof.  It has been a while, but I
> think only XFS really breaks this rule unless you overwrite the block size
> to 4K.  extn is fine.
> 
> The same rules apply to most SSDs.  Most SSDs prefer 4K alignment because of
> how the FTL (Flash Translation Layers) operate, even though <4K will
> sometimes still work pretty well.
> 
> Doug Dumitru
> CTO EasyCo LLC
> 
> On Mon, Dec 27, 2010 at 4:00 AM, D.S. Ljungmark <spider@skuggor.se> wrote:
> 
> > Hello,
> >  With the advent of the WD*EARS drives and the "advanced partitioning"
> > system that requires 4k blocks, I wanted to pop a quick question to see
> > how the md metadata is aligned and how it should be aligned to get
> > proper performance out of the devices.
> >
> > So, For raid1,  I'll assume there is some metadata-overhead on the
> > drives, how large is this block? Will there be need to make a partition
> > on the md-device in order to get proper alignment of filesystem =>
> > platters?
> >
> > for both raid1 and 5/6 levels, what are the appropiate stride-sizes for
> > ext3/4?
> >
> > For more advanced raid configurations (5-6), how should the ext3/4
> > stripe size be configured?
> >
> >
> > Yes, a lot of similarly naive questions, however, I'm asking mostly
> > because of how devices are changing from 512 to 4k sized blocks, with
> > quite interesting changes in performance, and I wanted to figure out
> > what is the current state of software raid. At which "level" of the
> > stack: (partition) raid (partition) filesystem ,  do you have to account
> > for the block sizes in order to not degrade performance of the devices.
> >
> > ( In a perfect world I'd be able to purchase a stack of the devices and
> > test for myself and come back with a report. However, money and hardware
> > is a scarce resource ;)
> >
> > ps.  please keep me CC'd as I'm not subscribed to the list.
> >
> >
> > // Spider
> >
> >
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> 
> 
>