Hi Luca, On Mon, April 20, 2009 15:46, Luca Berra wrote: > On Mon, Apr 20, 2009 at 03:15:12PM +0200, Sven Eschenberg wrote: >>Hi Luca, >> >>Okay, let's assume a chunk size of C. No matter what your md looks like, >>the logical md volume consists of a series of size/C chunks. the very >>first chunk C0 will hold the LVM header. >>If I align the extends with the chunksize and the extends even have the >>chunksize, then every extens PEx of my PV equals exactly a chunk on any >> of >>the disks. >>Which in turn means, if I want to read PEx I have to read some chunk Cy >> on >>one disk, and PEx+1 would most certainly be a Chunk Cy+1 which would >>reside on a different physical disk. > > correct > >>So the question is: Why would you want to align the first PE to the >>stripesize, rather then the chunksize? > > Because when you _write_ incomplete stripes, the raid code > would need to do a read-modify-write of the parity block. I didn't think of this 'yet', then again all the preliminary tests I did so far were on a 4D raid10 - Didn't have the time to setup the raid5 volume yet, because the performance issues on the raid10 were so amazing :-D. > > Filesystem, like ext3/4 and xfs have the ability to account for stripe > size in the block allocator to prevent unnecessary read-modify-writes, > but if you do not stripe-align the start of the filesystem you cannot > take advantage of this. > Since you mentioned it: What is the specific option (for xfs mainly) to modify this behavior? > The annoying issue is that rarely you have a (n^2)+P array, and pe_size > must be a power of 2. > So for example, given my 3D1P raid5 the only solution I devised was > having a chunk size which is a power of 2k, pe_start is aligned to > stripe, pe_size = chunk size, and I have to remember that every time I > extend a LV it has to be extended to the nearest multiple of 3 LE. Ouch, I see, I'm gonna be as lucky as you :-). Another question arose, when I thought about something: I actually wanted to place the OS on a stripe of mirrors, since this gives me the statistically best robustness against two failing disks. From what I could read in the md man page, non of the offered raid10 modes provides such a layout. Would I have to first mirror two drives with md and then stripe em together with md on top of md? > > Regards, > L. Regards -Sven > >>Regards >> >>-Sven >> >> >>On Mon, April 20, 2009 07:39, Luca Berra wrote: >>> On Sun, Apr 19, 2009 at 05:16:21PM +0200, Sven Eschenberg wrote: >>>> Unfortunately I don't have the box at hand for 2 days, but I asked md >>>> to >>>> use a chunksize of 2048K and the /proc/mdstat reported 2048K, last >>>> time >>>> I >>>> checked. >>>> The LVM hat a phy-extsize of 2M and with the --dataalignment option >>>> set >>>> to >>>> 2M, pvs reported a pe_start value of 2M aswell. >>> >>> if you have a 2M chunk size, a full stripe is 2M*(N-1), where N-1 is >>> the >>> number of drives in your array minus redundancy. (i.e. for a 5 drive >>> raid5 a stripe size would be 8M). >>> >>> L. >>> >>> -- >>> Luca Berra -- bluca@comedia.it >>> Communication Media & Services S.r.l. >>> /"\ >>> \ / ASCII RIBBON CAMPAIGN >>> X AGAINST HTML MAIL >>> / \ >>> >>> _______________________________________________ >>> linux-lvm mailing list >>> linux-lvm@redhat.com >>> https://www.redhat.com/mailman/listinfo/linux-lvm >>> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/ >>> >> >> >>_______________________________________________ >>linux-lvm mailing list >>linux-lvm@redhat.com >>https://www.redhat.com/mailman/listinfo/linux-lvm >>read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/ > > -- > Luca Berra -- bluca@comedia.it > Communication Media & Services S.r.l. > /"\ > \ / ASCII RIBBON CAMPAIGN > X AGAINST HTML MAIL > / \ > > _______________________________________________ > linux-lvm mailing list > linux-lvm@redhat.com > https://www.redhat.com/mailman/listinfo/linux-lvm > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/ > _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
