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Precise real-time discard in Reiser4 for SSD devices

Reiser4 possesses a unique feature - an efficient implementation of real-time discard which doesn't lead to garbage accumulation on disk, and, hence, rids of need to periodically run fstrim (batch discard) on the device.

Introduction. Garbage on SSD disks

Real-time discard support means that file system issues discard requests, i.e. informs the block layer about extents of freed space.

Currently all Linux file systems with announced feature of real-time discard support issue so-called "lazy" (or "non-precise") discard requests. It means that such file systems report exactly about blocks that were freed. Since erase units in general don't coincide with file system blocks, such "lazy" technique leads to accumulation of garbage on disk.

DEFINITION. Garbage is a set of erase units on disk, which were deallocated (marked free in the file system space map), but discard requests for them were not issued for some reasons.

Indeed, for example, if erase unit is larger than file system block, then it can happen that "lazy" discard request contains partial erase units, so that the block layer will round up the start and round down the end of such discard request. This is because on the one hand trim operation is defined only for whole erase units. On the other hand, the block layer doesn't know the status of erase unit, which is freed only partially, and hence it makes an assumption that it its other part is "busy" in the file system's space map (the alternative assumption can lead to data corruption). Note, however, that if such "forced" assumption is incorrect, and the whole erase unit becomes free, then such erase unit will become a garbage.

With lazy discard policy user needs once in a while to run special tools (e.g fstrim(8)) to clean up the accumulated garbage.

So, it would be nice to check the status of partially freed erase units and issue discard request for such unit, if its other part iDEFINITIONs also free (marked as free in the file system's space map). The block layer is not able to perform such checks for obvious reasons: this is a business of the file system. Below we prove that such checking of partially freed erase units and issuing discard requests for the padded extents (we'll call it "precise discard requests") doesn't lead to accumulation of garbage.

Efficient issuing precise discard requests without performance drop and ugly workarounds is possible only if the file system possesses an advanced transaction manager like the one of Reiser4.

Initial idea of precise discard and its implementation of complexity N_u (where N_u is total number of erase units (including partial ones) in the resulted set of sorted and merged discard requests) belongs to Ivan Shapovalov.

Edward Shishkin suggested implementation of complexity 2*N_e, where N_e is total number of extents in such resulted set.

(De)allocation, discard units and alignment. Non-precise and precise coordinates

The minimal unit of all (de)allocation operations in a file system is a file system block of blk_size.

The minimal unit of all discard operations is a so-called erase unit of EUS size.

Every file system block can be addressed by its (block) number. In this case we'll say about addressing in the system of non-precise coordinates 0Y.

In contrast with non-precise coordinates we'll also consider a system 0X of precise coordinates, where every individual byte on the disk can be addressed.

In the system 0Y we'll consider (non-precise) extents of blocks (U,V), where U is the number of the start block, and V is the width of the extent (in blocks).

In the system 0X we'll consider precise extents of bytes [AB], where A (A < B) is offset of the first byte and (B-1) is offset of the last byte of the extent. So, the length of such segment is B-A.

Erase unit size in bytes (EUS) is a property of SSD drive. Generally erase units don't coincide with file system blocks, so we'll address erase units in the system OX of precise coordinates by precise extents. In particular, every erase unit of some SSD partition is represented in precise coordinates as extent [EUO + N * EUS, EUO + (N+1)*EUS] for some natural N, where EUO is the offset of the first complete erase unit (0 <= EUO < EUS). That is, EUO is a property of individual partitions of SSD drives. EUO is also called as "alignment".

Lazy (non-precise) discard policy. Accumulation of garbage on disk

The policy of lazy (non-precise) discard is rather simple: if any extent of blocks (U,V) is freed by the file system, then we issue discard request for the extent [U * blk_size, (U+V) * blk_size].

Suppose now that EUS != blk_size, or EUO != 0

Suppose, the file system deallocates an extent (2,5) and issue the respective "lazy" discard request [2 * blk_size, 7 * blk_size], see the picture below. The block layer assumes that the neighboring blocks #1 and #7 are busy, and, hence, issues discard request for the smaller segment [AB].

Note, however, that if this assumption is incorrect, and blocks #1 and (or) #7 were actually free, then after freeing the extent (2,5) we'll have that the whole erase units [A - EUS, A] and [B, B + EUS] are marked as free in the file system space map, and hence, will replenish the garbage. So, the lazy (non-precise) discard policy leads to accumulation of garbage on disk.

*       *       *       *       *       *       *       *       *  > Y
0       1       2       3       4       5       6       7       8
0    blk_size      3*blk_size
*-------*-------*-------*-------*-------*-------*-------*-------*--> X
---+--------+--------+--------+--------+--------+--------+--------+> X
0 EUO     A-EUS      A                          B       B+EUS

Comment. There are 2 independent "sources" of garbage in "lazy" discard policy:

  • "bad" values of erase unit size (EUS != blk_size);
  • "bad" values of alignment (EUO != 0);

Precise discard policy

The idea is to check all "partially deallocated" erase units. If the whole such unit is marked as free in the file system space map, then we (file system) issue a discard request for the whole unit. That is, in contrast with the lazy discard policy, the file system provides correct status of every partially deallocated discard unit and issues precise discard request for the larger (padded) extents.

Let's consider the previous example. In accordance with the precise discard policy file system checks the status of blocks #1 and #7.

If both blocks #1 and #7 are free, then file system issues a discard request [A - EUS, B + EUS].

If block #1 is free and block #7 is busy, then file system issues a discard request [A - EUS, B].

If block #1 is busy and block #7 is free, then the file system will issue discard request [A, B + EUS].

Finally, if both blocks #1 and #7 are busy, then the file system issues discard request [A, B].

Note that block layer won't restrict such "precise" discard requests, and, moreover, the following statement takes place:

THEOREM. The policy of precise discard doesn't lead to accumulation of garbage on disk.

Proof (sketch). Indeed, suppose that disk doesn't contain "garbage". That is dicard request was issued for every erase unit, which is marked as free in the file system space map.

Suppose, the file system deallocates extent (2, 5).

If the block #1 is busy and block #7 is free, then, in accordance with precise discard policy, the file system issues "precise" discard request [A, B + EUS]. Note that we must not discard the unit [A - EUS, A], since it contains bytes of the busy block #1. Also, note that we don't need to discard other units due to the assumption, that before deallocation disk didn't contain garbage. Thus, we have that discard requests have been issued for every erase unit, which is marked as free in the file system space map.

By the similar way we can prove that "precise" discard policy doesn't leave garbage on disk in other 3 cases (when block #1 is free and block #7 is busy, both blocks #1 and #7 are free, and both blocks #1 and #7 are busy.

Implementation of precise discard

The straightforward solution is to check the status of partially deallocated erase units in the file system's space map. However, efficient implementation of such solution requires an advanced transaction manager and not less advanced block allocator. In particular, you need to make sure that nobody will occupy the other parts of your partially deallocated erase units while you are issuing precise discard requests for them (otherwise, data corruption is possible).

Reiser4 block allocator manages the following in-memory data-structures:

  • working space map (W)
  • commit space map (C)
  • deallocation set (D)

Allocation in Reise4 is always going on the working space map:

(1)   W' = alloc(W, R); - allocate a set R of block numbers in the working space map W.

Deallocation is a bit more complicated: all freed block numbers at first are recorded in a special data structure - deallocation set D:

(2)   D' = dealloc(D, R);

Before committing a transaction we update the commit space map C at so-called pre_commit_hook():

(3)   C' = apply(C, D);

After committing the transaction, that is after issuing all write requests (including the commit space map C) we prepare and issue discard requests in so-called post_write_back_hook():

(4)   prepare_and_issue_discard_requests();

After issuing discard requests we update the working space map:

(5)   W' = apply(W, D');

Handling paddings of partially freed erase units

When preparing discard set at stage (4) we check head (tail) padding of every partial erase unit. If it is free, we allocate it at the working space map:

W" = alloc(W', R');

At the same time we record the allocated paddings to the deallocation set:

D" = dealloc(D', R');

Updating the working space map at the stage (5) automatically deallocates the paddings:

apply(W", D") = W" \ R' = (W' + R')\ R'= W'.

How to test

Find out erase unit size and alignment of your SSD partition.

Apply the patch against reiser4-for-3.17.3.

Format a reiser4 partition with reiser4progs-1.0.9. Use mkfs.reiser4 option -d to "discard" the whole partition on your SSD drive at format time.

We recommend to use transparent compression for SSD drives (by default it is turned on when formatting with mkfs.reiser4).

Mount a reiser4 partition with mount option "discard", specifying (in bytes) erase unit size and alignment of your partition by mount options "discard.unit" and "discard.offset" respectively. For example, if erase unit size of your partition is 1536K and alignment is 0, then the mount command should look like the following:

mount /dev/sdX -o discard,discard.unit=1572864,discard.offset=0 /mnt

WARNING: Incorrectly specified erase unit size and alignment will lead to data corruption!

Find a kernel message about discard support:

reiser4: sdX: enable discard support (erase unit 1572864 bytes, alignment 0 bytes)

We recommend to use Write-Anywhere (AKA Copy-On-Write) transaction model for SSD drives (mount option "txmod=wa").

Also we recommend to use mount option "noatime" for SSD drives.

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