I understand now, thanks. RAID is more low level than filesystem, working at block device level, parity doesn't related to filesystem and file size at all, right? ----- Original Message ----- From: "Neil Brown" <neilb@xxxxxxx> To: "kyle" <kylewong@xxxxxxxxxx> Cc: <linux-raid@xxxxxxxxxxxxxxx> Sent: Saturday, November 12, 2005 7:47 PM Subject: Re: questions about ext3, raid-5, small files and wasted disk space > On Saturday November 12, kylewong@xxxxxxxxxx wrote: > > Hi, > > > > Assume I want to build a 8 x 250GB disks RAID-5, totally around 1.7TB, with > > ext3 file system, 4KB block size, and 32KB chunk size. > > Sounds like a reasonable configuration. > > > > > Almost half of the files will be very small ( < 8KB), I'm quite confused > > with how RAID-5 store data so I want to understand how relation between file > > system block size and RAID5 chunk size, to minimize possible wasted > > space. > > RAID-5 will always use one drive worth of space for parity. It > doesn't matter at all how the filesystem arranges data. > So in your config, the filesystem will appear to be about 1.7TB, and > you will have access to all of that space (or all that the file system > will allow you). > > > > > I understand that if I store a 224KB file into the RAID5, the file will be > > divided into 7 parts x 32KB, plus 32KB parity. (Am I correct in > > this?) > > Sort of ... if the filesystem happens to lay it out like that. But > this isn't a useful way to think about it. The filesystem writes the > data in 4K blocks. The raid5 layer worries about how to create the > parity block. > > > > > However, If I store a file smaller than chunk size into the RAID5, what will > > happen? For example, If I store a 1KB file into it, how much disk space > > will be used? will it 4KB (plus 4KB parity or 1KB parity) ? > > Well... if you store 7 4K files, there could be a parity block which > contains the xor-sum of all those files. But it depends on exactly > where the filesystem puts things. > > > > > Please CC me if possible, thanks. > ofcourse. > > > I hope that helps your understanding. > > NeilBrown > - 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
