Matthew Ahrens' Weblog

Is it magic? ZFS provides an impressive array of features, many of which are not available in traditional storage products. My partner-in-crime Mark Maybee has written about the utility of ZFS's quotas and reservaions. I'd like to talk about another features which we implemented, snapshots.

Snapshots

Snapshots are a familiar concept to many other filesystems: a snapshot is a view of a filesystem as it was at a particular point in time. ZFS's snapshots are useful in the same way that some other filesystems's snapshots are: By doing a backup of a snapshot, you have a consistent, non-changing target for the backup program to work with. Snapshots can also be used to recover from recent mistakes, by copying the fudged files from the snapshot.

What makes ZFS snapshots different is that we've removed all the limits. You can have as many snapshots as you want, taken as often as you like, and named as you choose. Taking a snapshot is a constant-time operation. The presence of snapshots doesn't slow down any operations. Deleting snapshots takes time proportional to the number of blocks that the delete will free, and is very efficient.

The awesome thing about ZFS being open source is that all these great properties of snapshots aren't magic -- you can see how it works for yourself! Snapshots are implemented in the DSL layer.

Background

Our entire filesystem is represented on disk as a giant tree of blocks with the leaf blocks containing data and the interior blocks containing metadata (mostly indirect blocks, but also dnode_phys_t's). To write a modified data block to disk, we write the modified data block to a new (previously unused) location on disk, rather than over-writing its existing location on disk. Now we must modify its parent block to point to the new on-disk location of the modified data block. We also write the modified parent block to a new (previously unused) location on disk. We continue this procedure of writing out parents to new locations until we get to the root of the tree, which is stored at a fixed location and is overwritten.

Data structures

Snapshots are a type of dataset, represented on-disk by a dsl_dataset_phys_t. To support snapshots, we maintain two data structures: a birth time associated with each block (blk_birth in the blkptr_t), and a list of "dead" blocks associated with each filesystem and snapshot (pointed to by ds_deadlist_obj in dsl_dataset_phys_t).

When writing the location of a block (the "block pointer") to its parent (eg. an indirect block), we also write the time that the child block was written -- the block's "birth time". This time is not literally the wall-clock time, but rather the value of a counter which increments each time we sync out a whole "transaction group" (aka "txg") and update the root of the block tree, the uberblock.

The dead list is an array of blkptr_t's which were referenced (or "live") in the previous snapshot, but are not referenced in this snapshot (or filesystem). These are the blocks that this snapshot "killed". I'll talk more about how this list is maintained and used in a bit.

Snapshot Creation

Conceptually, to take a snapshot, all we need to do is save the old uberblock before overwriting it, since it still points to valid, unmodified data. In fact, we do this not to the uberblock, but to the root of the sub-tree which represents a single filesystem, which is the objset_phys_t (or more generally, whatever ds_bp points to in dsl_dataset_phys_t). So each filesystem has its own snapshots, independent of other filesystems. The filesystem's snapshots are tracked in a doubly-linked list (the pointers are ds_prev_snap_obj and ds_next_snap_obj), sorted by the time they were taken, with the filesystem at the tail of the list. The snapshots also have administrator-chosen names, which are stored in a directory-like structure, maintained by the ZAP object pointed to by ds_snapames_zapobj.

When a snapshot is created, its dead list is set to the filesystem's dead list, and the filesystem's dead list is set to a new, empty list.

Snapshot creation happens in dsl_dataset_snapshot_sync().

Freeing blocks

When the filesystem is modified such that it no longer references a given block (eg. that block is overwritten, or the object that contains it is freed), the DMU will call dsl_dataset_block_born, which will determine whether we can actually free that block, reclaiming it storage space for other uses. We can free the block if and only if there are no other references to it. We can determine this by comparing the block's birth time (blk_birth) with the birth time of the most recent snapshot (ds_prev_snap_txg. If the block was born before the most recent snapshot, then that snapshot will reference the block and we can not free it. Otherwise, it was born after the most recent snapshot, and thus this snapshot (and all others) can not reference it, so we must free it.

When we can not free a block because the most recent snapshot was referencing it, we add its block pointer to the filesystem's dead list.

To summarize, there are two cases to consider when a block becomes no longer referenced by a filesystem:

                                   ---------------> time

block A:		    [... -----------------]

block B:				      [---]

[optional previous snapshots] ... ----- snap ------ fs

Block A was live when the most recent snapshot was taken, so that snapshot references it and this it can not be freed. Block B was born after the most recent snapshot, and thus no snapshots were taken of it so it must be freed.

Snapshot deletion

When a snapshot is deleted, dsl_dataset_destroy_sync will be called, which must determine which blocks we must free, and also maintain the dead lists. It's useful to think of 4 classes of blocks:

                                   ---------------> time

block A: [... --------------------------------]

block B:                       [--------------]

block C: [..................... ------------------------------------- ...]

block D: [... -------------------]

         ... ----- prev snap ----- this snap ------ next snap (or fs) ----- ...

To accomplish this, we iterate over the next snapshot's dead list (those in case A and B), and compare each block's birth time to the birth time of our previous snapshot. If the block was born before our previous snapshot (case A), then we do not free it, and we add it to our dead list. Otherwise, the block was born after our previous snapshot (case B), and we must free it. Then we delete the next snapshot's dead list, and set the next snapshot's dead list to our dead list. Finally, we can remove this snapshot from the linked list of snapshots, and the directory of snapshot names.

While the implementation is relatively simple, the algorithm is pretty subtle. How do we know that it is correct? First, did we free the correct blocks? The blocks we must free are those that are referenced by only the snapshot we are deleting (case B). Those blocks are the blocks which meet 4 constraints: (1) the were born after the previous snapshot, (2) the were born before this snapshot, (3) they died after this snapshot, and (4) they died before the next snapshot.

The blocks on the next snapshot's dead list are those that meet constraints (2) and (3) and (4) -- they are live in this snapshot, but dead in the next snapshot. (Note, the same applies if the next snapshot is actually the filesystem.) So to find the blocks that meet all constraints, we examine all the blocks on the next snapshot's dead list, and find those that meet constraint (1) -- ie. if the block's birth time is after the previous snapshot.

Now, did we leave the correct blocks on the next snapshot's dead list? This snapshot's dead list contains the blocks that were live in the previous snapshot, and dead in this snapshot (case D). If this snapshot did not exist, then they would be live in the previous snapshot and dead in the next snapshot, and therefore should be on the next snapshot's dead list. Additionally, the blocks which were live for both the previous snapshot and this snapshot, but dead in the next snapshot (case A) should be on the next snapshot's dead list.

Magic?

Hopefully this gives you a glimpse into how the DSL operates. For further reading, you might be interested in how zfs rollback works (see dsl_dataset_rollback_sync()). Clones are handled as a slightly special case of regular filesystems -- check out dsl_dataset_create_sync(). If you have any questions, don't hesitate to ask!

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Technorati Tag: OpenSolaris
Technorati Tag: Solaris
Technorati Tag: ZFS (2005-11-17 22:09:06.0) Permalink Comments [9]

Jeff Bonwick on 128-bits Fellow ZFS engineer Jeff Bonwick has posted a blog entry on why 128 bit storage addresses are necessary and sufficient. (2004-09-27 10:38:57.0) Permalink Comments [3]

ZFS featured on sun.com homepage The sun.com homepage features a new article on ZFS. Check it out, and feel free to post comments with the form at the bottom of the article, on this blog, or at the Expert Exchange tomorrow. (2004-09-14 15:36:10.0) Permalink Comments [7]

Expert Exchange (chat session) with ZFS engineers Sun is hosting an Expert Exchange on ZFS. Basically this is an interactive chat session where you can ask fellow ZFS engineer Bill Moore and I questions about ZFS. The session will be this Wednesday, September 15 from 10-11am PDT. You can find more information here.

A transcript of the chat session is now available (click on 'archives' from the expert exchange website). (2004-09-13 17:46:25.0) Permalink Comments [5]

What is ZFS? It occurs to me that before I can really talk much about ZFS, you need to know what it is, and how it's generally arranged. So here's an overview of what ZFS is, reproduced from our internal webpage:

ZFS is a vertically integrated storage system that provides end-to-end data integrity, immense (128-bit) capacity, and very simple administration.

To applications, ZFS looks like a standard POSIX filesystem. No porting is required.

To administrators, ZFS presents a pooled storage model that completely eliminates the concept of volumes and the associated problems of partition management, provisioning, and filesystem grow/shrink. Thousands or even millions of filesystems can all draw from a common storage pool, each one consuming only as much space as it actually needs. Moreover, the combined I/O bandwidth of all devices in the pool is available to all filesystems at all times.

All operations are copy-on-write transactions, so the on-disk state is always valid. There is no need to fsck(1M) a ZFS filesystem, ever.

Every block is checksummed to prevent silent data corruption, and the data is self-healing in mirrored or RAID configurations. When one copy is damaged, ZFS detects it (via the checksum) and uses another copy to repair it.

ZFS provides an unlimited number of snapshots, which are created in constant-time, and do not require additional copies of any data. Snapshots provide point-in-time copies of the data for backups and end-user recovery from fat-finger mistakes.

ZFS provides clones, which provide a fast, space-efficient way of making "copies" of filesystems. Clones are extremely useful when many almost-identical "copies" of a set of data are required -- for example, multiple code sourcebases, one for each engineer or bug being fixed; or multiple system images, one for each zone or netboot-ed machine.

ZFS also provides quotas, to limit space consumption; reservations, to guarantee availability of space in the future; compression, to reduce both disk space and I/O bandwidth requirements; and supports the full range of NFSv4/NT-style ACLs.

What exactly is a "pooled storage model"? Basically it means rather than stacking one FS on top of one volume on top of some disks, you stack many filesystems on top of one storage pool on top of lots of disks. Take a home directoy server with a few thousand users and a few terabytes of data. Traditionally, you'd probably set it up so that there are a few filesystems, each a few hundred megabytes, and put a couple hundred users on each filesystem.

That seems odd -- why is there an arbitrariy grouping of users into filesystems? It would be more logical to have either one filesystem for all users, or one filesystem for each user. We can rule out the latter because it would require that we statically partition our storage and decide up front how much space each user got -- ugh. Using one big filesystem would be plausable, but performance may become a problem with large filesystems -- both common run-time performance and performance of administrative tasks. Many backup tools are filesystem-based. The run-time of fsck(1m) is not linear in the size of the filesystem, so it could take a lot longer to fsck that one 8TB filesystem than it would to fsck 80 100GB filesystems. Furthermore, some filesystems simply don't support more than a terabyte or so of storage.

It's inconvenient to run out of space in a traditional filesystem, and happens all too often. You might have lots of free space in a different filesystem, but you can't easily use it. You could manually migrate users to different filesystems to balance the free space... (hope your users don't mind downtime! hope you find the right backup tape when it comes time to restore!) Eventually you'll have to install new disks, make a new volume and filesystem out of them, and then migrate some users over to the new filesystem, incurring downtime. I experienced these kinds of problems with my home directory when I was attending school (using VxFS on VxVM on Solaris), they still plague some home directory and other file servers at Sun (using UFS on SVM on Solaris).

With ZFS, you can have one storage pool which encompasses all the storage attached to your server. Then you can easily create one filesystem for each user. When you run low on storage, simply attach more disks and add them to the pool. No downtime. This is the scenario on the home directory server that I use at Sun, which uses ZFS on Solaris.

Thus concludes ZFS lesson 1. (2004-08-30 15:38:27.0) Permalink Comments [33]

Welcome I am an engineer in the Solaris kernel group. I've been working for Sun for the past three years, primarily on ZFS, the Zettabyte File System. Expect posting frequency to increase as ZFS becomes increasingly available to the outside world. (2004-07-28 13:49:19.0) Permalink Comments [7]