ARC should cache compressed data
When reading compressed data from disk, the ARC should keep the compressed block cached and only decompress it when consumers access the block. The uncompressed data should be short-lived allowing the ARC to cache a much larger amount of data. The DMU would also maintain a smaller cache of uncompressed blocks to minimize the impact of decompressing frequently accessed blocks.
Updated by Electric Monk about 3 years ago
- Status changed from In Progress to Closed
- % Done changed from 0 to 100
commit dcbf3bd6a1f1360fc1afcee9e22c6dcff7844bf2 Author: George Wilson <firstname.lastname@example.org> Date: 2016-07-14T18:55:57.000Z 6950 ARC should cache compressed data Reviewed by: Prakash Surya <email@example.com> Reviewed by: Dan Kimmel <firstname.lastname@example.org> Reviewed by: Matt Ahrens <email@example.com> Reviewed by: Paul Dagnelie <firstname.lastname@example.org> Reviewed by: Don Brady <email@example.com> Reviewed by: Richard Elling <Richard.Elling@RichardElling.com> Approved by: Richard Lowe <firstname.lastname@example.org>
Updated by Joshua M. Clulow 8 months ago
From the commit message in the original commit to the OpenZFS repository:
This change covers the reading and writing of compressed arc headers, sharing data between the arc_hdr_t and the arc_buf_t, and the implementation of a new dbuf cache to keep frequently access data uncompressed.
I've added a new member to l1 arc hdr called b_pdata. The b_pdata always hangs off the arc_buf_hdr_t (if an L1 hdr is in use) and points to the physical block for that DVA. The physical block may or may not be compressed. If compressed arc is enabled and the block on-disk is compressed, then the b_pdata will match the block on-disk and remain compressed in memory. If the block on disk is not compressed, then neither will the b_pdata. Lastly, if compressed arc is disabled, then b_pdata will always be an uncompressed version of the on-disk block.
Typically the arc will cache only the arc_buf_hdr_t and will aggressively evict any arc_buf_t's that are no longer referenced. This means that the arc will primarily have compressed blocks as the arc_buf_t's are considered overhead and are always uncompressed. When a consumer reads a block we first look to see if the arc_buf_hdr_t is cached. If the hdr is cached then we allocate a new arc_buf_t and decompress the b_pdata contents into the arc_buf_t's b_data. If the hdr already has a arc_buf_t, then we will allocate an additional arc_buf_t and bcopy the uncompressed contents from the first arc_buf_t to the new one.
Writing to the compressed arc requires that we first discard the b_pdata since the physical block is about to be rewritten. The new data contents will be passed in via an arc_buf_t (uncompressed) and during the I/O pipeline stages we will copy the physical block contents to a newly allocated b_pdata.
When an l2arc is inuse it will also take advantage of the b_pdata. Now the l2arc will always write the contents of b_pdata to the l2arc. This means that when compressed arc is enabled that the l2arc blocks are identical to those stored in the main data pool. This provides a significant advantage since we can leverage the bp's checksum when reading from the l2arc to determine if the contents are valid. If the compressed arc is disabled, then we must first transform the read block to look like the physical block in the main data pool before comparing the checksum and determining it's valid.