[CentOS] storage for mailserver

Wed Sep 16 16:48:19 UTC 2020
Valeri Galtsev <galtsev at kicp.uchicago.edu>

On 2020-09-16 11:26, Stephen John Smoogen wrote:
> On Wed, 16 Sep 2020 at 12:12, Michael Schumacher <
> michael.schumacher at pamas.de> wrote:
>> hi,
>> I am planning to replace my old CentOS 6 mail server soon. Most details
>> are quite obvious and do not need to be changed, but the old system
>> was running on spinning discs and this is certainly not the best
>> option for todays mail servers.
>> With spinning discs, HW-RAID6 was the way to go to increase reliability
>> and speed.
>> Today, I get the feeling, that traditional RAID is not the best
>> option for SSDs. I am reading that all RAID members in SSD-arrays age
>> synchronously so that the risk of a massive failure of more than one
>> disk is more likely than with HDDs. There are many other concerns like
>> excessive write load compared to non-raid systems, etc.
>> Is there any common sense what disk layout should be used these days?
>> I have been looking for some kind of master-slave system, where the
>> (one or many) SSD is taking all writes and reads, but the slave HDD
>> runs in parallel as a backup system like in a RAID1 system. Is there
>> any such system?
>> I don't think so because the drives would always be out of sync but in a
> restart it would be hard to know if the drive is out of sync for a good
> reason or a bad one. For most of the SSD raids, I have seen people just
> making sure to buy disks which are spec'd for more writes or similar
> 'smarter' enterprise trim. I have also read about the synchronicity problem
> but I think this may be a theory vs reality problem. In theory they should
> all fail at once, in reality at least for the arrays I have used for 3
> years, they seem to fail in different times. that said, I only have 3
> systems over 3 years with SSD drives running RAID6 so I only have anecdata
> versus data.

I fully agree about synchronous failure of SSDs in RAID to be made up or 
grossly overrated. SSD failure _probablity_ is increased with number of 
write operations (into the same area). Failure still has stochastic 
nature. If SSD is spec'ed for N number of writes, it doesn't mean on the 
write N+1 SSD will fail. It only means that after N number of writes 
failure probability is below [some acceptable value], which, however is 
much higher of that of unused SSD.

That said, single SSD failure probability after long run is some small 
value, say q. Event of failure of another SSD is independent event from 
failure of first failed SSD (even though their probabilities q both 
increase with number of writes) hence probability of failures are:

one SSD failed:  q

two SSDs failed: (q)^2

three SSDs failed: (q)^3

thus multi-failures (say, within some period of time, say 1 day, or 1 
week) still are way less probable events than single failure. The 
following numbers have nothing to do with probability of failure of some 
devices, it is just an illustration, so:

if q = 10 ^ (-10) (ten to the minus 10th power), then

(q)^2 = 10 ^ (-20)

(q)^3 = 10 ^ (-30)

My apologies for saying trivial things, they just give IMHO a feeling of 
what to take into consideration, and what to ignore safely.

And no, I don't intend to start flame war on views of statistics, or on 
hardware vs software RAIDs, or RAIDs vs zfs. Just think it over and draw 
your own conclusions.


>> Any thoughts?
>> best regards
>> Michael Schumacher
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Valeri Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247