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From "Jon Haddad (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-8460) Make it possible to move non-compacting sstables to slow/big storage in DTCS
Date Mon, 26 Mar 2018 20:38:00 GMT


Jon Haddad commented on CASSANDRA-8460:

Hey [~Lerh Low], sorry for the delay.  I've been running some tests on lvm cache using fio
for benchmarking rather than C*.  Cassandra adds a layer of complexity that won't help when
it comes to raw benchmarks.

I ran some tests on EBS, SSD (i2.large), and EBS using SSD as a cache volume.  I ran with
this simple configuration to start:




||Metric||EBS||SSD||EBS + Cache||
|Random Read IOPS|1509|5748|5347|
|Random Read Bandwidth|6MB/s|22MB/s|21MB/s|
|Seq Write IOPS|40K|145K|39K|
|Seq Write Bandwidth|163MB/s|580MB/s|156MB/s|

I've set up the cache as writethrough, meaning we're going to be bottlenecked on the slow
disk for writes.  Here's the setup:

root@ip-172-31-45-143:~# lvs -a
  LV              VG   Attr       LSize   Pool    Origin         Data%  Meta%  Move Log Cpy%Sync
  [cache]         test Cwi---C--- 700.00g                        7.25   0.55            0.00
  [cache_cdata]   test Cwi-ao---- 700.00g
  [cache_cmeta]   test ewi-ao----  40.00g
  [lvol0_pmspare] test ewi-------  40.00g
  origin          test Cwi-aoC---   1.50t [cache] [origin_corig] 7.25   0.55            0.00
  [origin_corig]  test owi-aoC---   1.50t

Generally speaking, TWCS uses considerably less I/O than any other strategy, and it works
fine with spinning disks on EBS already, so I'm inclined to _personally_ lean towards using
LVM.  It doesn't require any additional configuration once the volume is set up, and as I
mentioned previously it's been baked into the Linux kernel for a long time now.  I haven't
researched what's available on Windows, so that's something to keep in mind.

I'm not opposed to research, or new features, but this seems to be to me to be adding complexity
to solve a problem that's already been solved.  

> Make it possible to move non-compacting sstables to slow/big storage in DTCS
> ----------------------------------------------------------------------------
>                 Key: CASSANDRA-8460
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Marcus Eriksson
>            Assignee: Lerh Chuan Low
>            Priority: Major
>              Labels: doc-impacting, dtcs
>             Fix For: 4.x
> It would be nice if we could configure DTCS to have a set of extra data directories where
we move the sstables once they are older than max_sstable_age_days. 
> This would enable users to have a quick, small SSD for hot, new data, and big spinning
disks for data that is rarely read and never compacted.

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