NeilBrown put forth on 3/24/2011 1:33 AM: > On Thu, 24 Mar 2011 00:52:00 -0500 Stan Hoeppner <stan@xxxxxxxxxxxxxxxxx> > wrote: > >> If you write a file much smaller than the stripe size, say a 1MB file, >> to the filesystem atop this wide RAID10, the file will only be striped >> across 16 of the 192 spindles, with 64KB going to each stripe member, 16 >> filesystem blocks, 128 sectors. I don't know about mdraid, but with >> many hardware RAID striping implementations the remaining 176 disks in >> the stripe will have zeros or nulls written for their portion of the >> stripe for this file that is a tiny fraction of the stripe size. > > This doesn't make any sense at all. No RAID - hardware or otherwise - is > going to write zeros to most of the stripe like this. The RAID doesn't even > know about the concept of a file, so it couldn't. > The filesystem places files in the virtual device that is the array, and the > RAID just spreads those blocks out across the various devices. > > There will be no space wastage. Well that's good to know then. Apparently I was confusing partial block writes with partial stripe writes. Thanks for clarifying this Neil. > If you have a 1MB file, then there is no way you can ever get useful 192-way > parallelism across that file. That was exactly my point. Hence my recommendation against very wide stripe arrays for general purpose fileservers. > Bit if you have 192 1MB files, then they will > be spread even across your spindles some how (depending on FS and RAID level) > and if you have multiple concurrent accessors, they could well get close to > 192-way parallelism. The key here being parallelism, to a great extent. All 192 files would need to be in the queue simultaneously. This would have to be a relatively busy file or DB server. -- Stan -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
