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
