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diff --git a/IoTunables.mdwn b/IoTunables.mdwn new file mode 100644 index 0000000..42e045c --- /dev/null +++ b/IoTunables.mdwn @@ -0,0 +1,91 @@ +IO Tunables, Options, and Knobs + +## What there is + +These IO tunables can be set on a per-filesystem basis in +`/sys/fs/bcache/<FSUUID>`. Some can be overridden on a per-inode basis via +extended attributes; the xattr name will be listed in such cases. + +### Metadata tunables + +- `metadata_checksum` + - What checksum algorithm to use for metadata + - Default: `crc32c` + - Valid values: `none`, `crc32c`, `crc64` +- `metadata_replicas` + - The number of replicas to keep for metadata + - Default: `1` +- `metadata_replicas_required` + - The minimum number of replicas to tolerate for metadata + - Default: `1` +- `str_hash` + - The hash algorithm to use for dentries and xattrs + - Default: `siphash` + - Valid values: `crc32c`, `crc64`, `siphash` + +### Data tunables + +- `data_checksum` + - What checksum algorithm to use for data + - Extended attribute: `bcachefs.data_checksum` + - Default: `crc32c` + - Valid values: `none`, `crc32c`, `crc64` +- `data_replicas` + - The number of replicas to keep for data + - Extended attribute: `bcachefs.data_replicas` + - Default: `1` +- `data_replicas_required` + - The minimum number of replicas to tolerate for data + - Default: `1` + +### Foreground tunables + +- `compression` + - What compression algorithm to use for foreground writes + - Extended attribute: `bcachefs.compression` + - Default: `none` + - Valid values: `none`, `lz4`, `gzip`, `zstd` +- `foreground_target` + - Extended attribute: `bcachefs.foreground_target` + - What disk group foreground writes should prefer (may use other disks if + sufficient replicas are not available in-group) + +### Background tunables + +- `background_compression` + - What compression algorithm to recompress to in the background + - Extended attribute: `bcachefs.background_compression` + - Default: `none` + - Valid values: `none`, `lz4`, `gzip`, `zstd` +- `background_target` + - Extended attribute: `bcachefs.background_target` + - What disk group data should be written back to in the background (may + use other disks if sufficient replicas are not available in-group) + +### Promote (cache) tunables + +- `promote_target` + - What disk group data should be copied to when frequently accessed + - Extended attribute: `bcachefs.promote_target` + +## What would be nice + +- Different replication(/raid) settings between FG, BG, and promote + - For example, replicated foreground across all SSDs (durability with low + latency), RAID-6 background (sufficient durability, optimal space + overhead per durability), and RAID-0/single promote (maximum performance, + minimum cache footprint) + +- Different compression for promote + - For example, uncompressed foreground (minimize latency), zstd background + (minimize size), and lz4 promote (save cache space, but prioritize speed) + +- Fine-grained tuning of stripes vs. replicas vs. parity? + - Would allow configuring N stripes + M parity, so trading off throughput + (more stripes) vs. durability (more parity or replicas) vs. space + efficiency (fewer replicas, higher stripes/parity ratio) + - Well defined: (N stripes, R replicas, P parity) = (N * R + P) total + chunks, with each stripe replicated R times, and P parity chunks to + recover from if those are insufficient. + - Can be implemented with P <= 2 for now, possibly extended later using + Andrea Mazzoleni's compatible approach (see [[Todo]], wishlist section) diff --git a/sidebar.mdwn b/sidebar.mdwn index 2cffc86..5c20c3b 100644 --- a/sidebar.mdwn +++ b/sidebar.mdwn @@ -7,6 +7,7 @@ * [[Contact]] * Development * [[Todo]] + * [[IoTunables]] * Bcachefs developer documentation * [[Architecture]] * [[Encryption]] |