Re: [zfs-discuss] Basic question about striping and ZFS
Kjetil Torgrim Homme writes: > Cindy Swearingen writes: >> You might check the slides on this page: >> >> http://hub.opensolaris.org/bin/view/Community+Group+zfs/docs >> >> Particularly, slides 14-18. >> >> In this case, graphic illustrations are probably the best way >> to answer your questions. > > thanks, Cindy. can you explain the meaning of the blocks marked X in > the illustration on page 18? I found the explanation in an older (2009-09-03) message to this list from Adam Leventhal: | RAID-Z writes full stripes every time; note that without careful | accounting it would be possible to effectively fragment the vdev | such that single sectors were free but useless since single-parity | RAID-Z requires two adjacent sectors to store data (one for data, | one for parity). To address this, RAID-Z rounds up its allocation to | the next (nparity + 1). This ensures that all space is accounted | for. RAID-Z will thus skip sectors that are unused based on this | rounding. For example, under raidz1 a write of 1024 bytes would | result in 512 bytes of parity, 512 bytes of data on two devices and | 512 bytes skipped. -- Kjetil T. Homme Redpill Linpro AS - Changing the game ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] Basic question about striping and ZFS
So then of what use is the parity? And how is the metadata used to reconstruct bad data? I understand obviously what the metadata contains but I don't get how ZFS traverses through a file system and USES the metadata to construct bad blocks. I understand that you write everything to separate blocks. My question was this: If you have initially two stripes over two disks like this: Disk 1: (Stripe Unit 1) Disk 2: (Stripe Unit 2) You then want to modify something in the first stripe unit with modifications which are smaller so now Disk 1 and Disk 2 stripes look like this: Disk 1: XXYY (the y's indicate modified bits or bytes or whatever) Disk 2: So now, with a full-stripe write, you then make new blocks for both stripes and just copy the data over to the new blocks. Now, tell me if I am write with what happens on a full-stripe write: You read in Disk 1 and Disk 2 stripes in the file system cache. You then apply the modifications to the Disk 1 stripe within the cache. After this, you compute the parity within the cache and finally you write out both Disk 1 Stripe and Disk 2 stripe to new blocks. Since the modifications to the disk 1 stripe (the Ys) were smaller than the total stripe size, the new sector which will be written to will be of a smaller stripe size than the originals. Is this correct? -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] Basic question about striping and ZFS
On Thu, Nov 05, 2009 at 11:55:58AM -0800, Ilya wrote: > Slide 18 shows variably sizes extents but doesn't explain the process > of full-on write. What I'm looking for is one example. I still don't > understand how it works with variable sized extents. So if you have 2 > stripe units on one disk and 1 stripe unit for the parity and you > modify half of the first stripe unit only, when you do a full-stripe > write, what happens in terms of a full-stripe write? You never modify a ZFS block (or part of a ZFS block). You write a new replacement block elsewhere and a new metadata tree is constructed that references the new block and not the old one (other than via snapshots). I'm not sure I understand your picture of "2 stripe units on one disk and 1 stripe unit for parity". That doesn't seem correct. Are you looking at a particular portion of that graphic that you could reference? > I also didn't see a distinction between parity and metadata > reconstruction. I still do not know the process behind the metadata > reconstruction for bad data and when parity is used for bad data. Not sure what you mean by metadata reconstruction. The checksums stored in parent blocks can be used to validate child blocks (either metadata or data). If the checksum fails, and there is redundant information (copy, parity, mirror), then the system tries to see if the data is available through the redundant data. It will read the other half of the mirror, or try to read a data using a parity reconstruction. It will then validate that other read via the checksum. If that checksum succeeds, you've read the data and the system should attempt to rewrite the redundant info (assuming it was a bad block and not a disk failure that has left the pool in a degraded state). If the checksum fails and there is no redundant copy, then the data is not returned. -- Darren ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] Basic question about striping and ZFS
Hey Thanks for the slides but some things are still unclear. Slide 18 shows variably sizes extents but doesn't explain the process of full-on write. What I'm looking for is one example. I still don't understand how it works with variable sized extents. So if you have 2 stripe units on one disk and 1 stripe unit for the parity and you modify half of the first stripe unit only, when you do a full-stripe write, what happens in terms of a full-stripe write? I also didn't see a distinction between parity and metadata reconstruction. I still do not know the process behind the metadata reconstruction for bad data and when parity is used for bad data. -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] Basic question about striping and ZFS
Forgot to add, are those four stripe units (for that one file) above considered the stripe itself? Or are each of those stripe units on the seperate disks considered as separate stripes? -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] Basic question about striping and ZFS
Researching about ZFS and had a question leating to Raid-Z and the striping. So, I was glacing over Jeff's blog (http://blogs.sun.com/bonwick/entry/raid_z): [i]"RAID-Z is a data/parity scheme like RAID-5, but it uses dynamic stripe width. Every block is its own RAID-Z stripe, regardless of blocksize. This means that every RAID-Z write is a full-stripe write. This, when combined with the copy-on-write transactional semantics of ZFS, completely eliminates the RAID write hole. RAID-Z is also faster than traditional RAID because it never has to do read-modify-write. "[/i] So firstly, is this literally referring to the blocks of a file for example? Also by stripe, is this referring to the stripe UNITS (within a whole stripe) or the ENTIRE stripe across disks? So, let's say that you have a file of 64 kb per sector (stripe units consisting of blocks of whatever size totaling 64k) across four disks. Disk 0: Stripe 1 Disk 1: Stripe 2 Disk 2: Stripe 3 Disk 3: Parity When Jeff's blog mentions that "every block has it's own stripe" what does he exactly mean in the context of this example? And let's say that I am modifying/write out bytes in the first stripe, how does this affect the other stripes/parity? -- This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
