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https://issues.apache.org/jira/browse/KUDU-1489?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15518111#comment-15518111
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Todd Lipcon commented on KUDU-1489:
-----------------------------------
More than just switching to the WAL dir, we may want to allow striping across
all of the disks, given that many users don't have SSDs. Striping across 12
spinning disks gives a 12x speedup vs trying to fit them all into one.
A related idea mentioned in the doc attached to KUDU-1635 is to store cmeta for
a single tablet on multiple drives, and do latency-leveling across them (write
to whichever one is faster).
> Use WAL directory for tablet metadata files
> -------------------------------------------
>
> Key: KUDU-1489
> URL: https://issues.apache.org/jira/browse/KUDU-1489
> Project: Kudu
> Issue Type: Bug
> Components: fs, tserver
> Affects Versions: 0.9.0
> Reporter: Adar Dembo
> Priority: Critical
>
> Today a tserver will place tablet metadata files (i.e. superblock and cmeta
> files) in the first configured data directory. I don't remember why we
> decided to do this (commit 691f97d introduced it), but upon reconsideration
> the WAL directory seems like a much better choice, because if the machine has
> different kinds of I/O devices, the WAL directory's device is typically the
> fastest.
> Mostafa has been testing Impala and Kudu on a cluster with many thousands of
> tablets. His cluster contains storage-dense machines, each configured with 14
> spinning disks and one flash device. Naturally, the WAL directory sits on
> that flash device and the data directories are on the spinning disks. With
> thousands of tablet metadata files on the first spinning disk, nearly every
> tablet in the tserver is bottlenecked on that device due to the sheer amount
> of I/O needed to maintain the running state of the tablet, specifically
> rewriting cmeta files on various Raft events (votes, term advancement, etc.).
> Many thousands of tablets is not really a good scale for Kudu right now, but
> moving the tablet metadata files to a faster device should at least help with
> the above.
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