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From Leif Hedstrom <zw...@apache.org>
Subject Re: persistent disk write at rate of ~4 MB/s
Date Sun, 19 Oct 2014 19:19:48 GMT

> On Oct 19, 2014, at 12:47 PM, Timur Irmatov <irmatov@gmail.com> wrote:
> 
> On Sun, Oct 19, 2014 at 8:01 PM, Leif Hedstrom <zwoop@apache.org> wrote:
>> That sounds like the dir sync task. You can try changing the config to sync less
frequently and see if that affects it? On very large disks it probably has to get very long,
basically the dir sync throttles itself to not put too much pressure on the disks. Maybe we
should make that configurable too.
> 
> What is dir sync task? Could you elaborate? I haven't found any
> descriptions of that nor configuration settings.

  //  # how often should the directory be synced (seconds)
  {RECT_CONFIG, "proxy.config.cache.dir.sync_frequency", RECD_INT, "60", RECU_DYNAMIC, RR_NULL,
RECC_NULL, NULL, RECA_NULL}
  ,

> 


So, traffic_server keeps an in-memory lookup table of disk cache items. In order to guarantee
consistency, this RAM table has to be synced to disk periodically. You can reduce write pressure
by increasing this sync frequency; However, be aware that doing so increases the window where
you can lose disk data. This is safe, just means you could lose more data from showing up
as written to disk during restarts.

— Leif


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