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List:       ceph-users
Subject:    [ceph-users] Re: High ceph_osd_commit_latency_ms on Toshiba MG07ACA14TE HDDs
From:       vitalif () yourcmc ! ru
Date:       2020-06-24 21:51:39
Message-ID: a8a42d0df3aa751d57a8b679c2a0a4e5 () yourcmc ! ru
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Hi, https://yourcmc.ru/wiki/Ceph_performance author here %)

Disabling write cache is REALLY bad for SSDs without capacitors 
[consumer SSDs], also it's bad for HDDs with firmwares that don't have 
this bug-o-feature. The bug is really common though. I have no idea 
where it comes from, but it's really common. When you "disable" the 
write cache you actually "enable" the non-volatile write cache on those 
drives. Seagate EXOS drives also behave like that... It seems most EXOS 
drives have an SSD cache even though it's not mentioned in specs. And it 
gets enabled when you do hdparm -W 0. In theory hdparm -W 0 may hurt 
linear write performance even on those HDDs, though.

> Well, what I was saying was "does it hurt to unconditionally run hdparm 
> -W
> 0 on all disks?"
> 
> Which disk would suffer from this? I haven't seen any disk where this 
> would
> be a bad idea
> 
> Paul
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