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Documentation → Configuration → Cluster-Wide Disk Layout Parameters
Cluster-Wide Disk Layout Parameters
These parameters apply to clients and OSDs, are fixed at the moment of OSD drive initialization and can't be changed after it without losing data.
OSDs with different values of these parameters (for example, SSD and SSD+HDD OSDs) can coexist in one Vitastor cluster within different pools. Each pool can only include OSDs with identical settings of these parameters.
These parameters, when set to a non-default value, must also be specified in etcd for clients to be aware of their values, either in /vitastor/config/global or in pool configuration. Pool configuration overrides the global setting. If the value for a pool in etcd doesn't match on-disk OSD configuration, the OSD will refuse to start PGs of that pool.
block_size
- Type: integer
- Default: 131072
Size of objects (data blocks) into which all physical and virtual drives (within a pool) are subdivided in Vitastor. One of current main settings in Vitastor, affects memory usage, write amplification and I/O load distribution effectiveness.
Recommended default block size is 128 KB for SSD and 4 MB for HDD. In fact, it's possible to use 4 MB for SSD too - it will lower memory usage, but may increase average WA and reduce linear performance.
OSD memory usage is roughly (SIZE / BLOCK * 68 bytes) which is roughly 544 MB per 1 TB of used disk space with the default 128 KB block size.
bitmap_granularity
- Type: integer
- Default: 4096
Required virtual disk write alignment ("sector size"). Must be a multiple of disk_alignment. It's called bitmap granularity because Vitastor tracks an allocation bitmap for each object containing 2 bits per each (bitmap_granularity) bytes.
Can't be smaller than the OSD data device sector.
immediate_commit
- Type: string
- Default: false
Another parameter which is really important for performance.
Desktop SSDs are very fast (100000+ iops) for simple random writes without cache flush. However, they are really slow (only around 1000 iops) if you try to fsync() each write, that is, when you want to guarantee that each change gets immediately persisted to the physical media.
Server-grade SSDs with "Advanced/Enhanced Power Loss Protection" or with "Supercapacitor-based Power Loss Protection", on the other hand, are equally fast with and without fsync because their cache is protected from sudden power loss by a built-in supercapacitor-based "UPS".
Some software-defined storage systems always fsync each write and thus are really slow when used with desktop SSDs. Vitastor, however, can also efficiently utilize desktop SSDs by postponing fsync until the client calls it explicitly.
This is what this parameter regulates. When it's set to "all" the whole Vitastor cluster commits each change to disks immediately and clients just ignore fsyncs because they know for sure that they're unneeded. This reduces the amount of network roundtrips performed by clients and improves performance. So it's always better to use server grade SSDs with supercapacitors even with Vitastor, especially given that they cost only a bit more than desktop models.
There is also a common SATA SSD (and HDD too!) firmware bug (or feature)
that makes server SSDs which have supercapacitors slow with fsync. To check
if your SSDs are affected, compare benchmark results from fio -name=test -ioengine=libaio -direct=1 -bs=4k -rw=randwrite -iodepth=1 with and without
-fsync=1. Results should be the same. If fsync=1 result is worse you can
try to work around this bug by "disabling" drive write-back cache by running
hdparm -W 0 /dev/sdXX or echo write through > /sys/block/sdXX/device/scsi_disk/*/cache_type
(IMPORTANT: don't mistake it with /sys/block/sdXX/queue/write_cache - it's
unsafe to change by hand). The same may apply to newer HDDs with internal
SSD cache or "media-cache" - for example, a lot of Seagate EXOS drives have
it (they have internal SSD cache even though it's not stated in datasheets).
Setting this parameter to "all" or "small" in OSD parameters requires enabling disable_journal_fsync and disable_meta_fsync, setting it to "all" also requires enabling disable_data_fsync.
TLDR: For optimal performance, set immediate_commit to "all" if you only use SSDs with supercapacitor-based power loss protection (nonvolatile write-through cache) for both data and journals in the whole Vitastor cluster. Set it to "small" if you only use such SSDs for journals. Leave empty if your drives have write-back cache.