Re: [MooseFS-Users] Fwd: CHUNKS_LOOP_MAX_CPS clarification

[Marco M. <mar...@gm...>]

On 05/11/2018 03:30 AM, Gandalf Corvotempesta wrote:
> Il giorno ven 11 mag 2018 alle ore 03:26 Marco Milano <mar...@gm...>
> ha scritto:
>> It is also checked at every read. If the CRC doesn't match,
>> it will self-heal from the good copy.
>> So, CRC is not useless. CRC testing is an additional layer of protection
>> on top of "check CRC on every read". It is not the "only" protection.
> 
> Is not the same.
> If you have a huge storage, with mostly unaccessed data, like an archive
> storage or a backup storage,
> most of stored data won't be accessed for year and you won't be able to
> detect if something wrong is happening
> on you disks.
> 
> It's like with any RAID, every RAID is able to detect read errors and act
> accordingly by reading from a different replica or by reconstructing data
> from the parity.
> But without a monthly/weekly scrub of the whole array, what would happen if
> you have a bad sector affecting a file that you haven't read from month,
> and then
> a disk fails? When you are rebuilding the failed disk, you'll get the URE
> on the "good" disk, resulting in data loss (data can't be reconstructed,
> because the "good" disk as an unreadable sector) and a punctured raid
> (happened to me many times in the past)
> 
The scenario you describe is only likely to happen if you have only two copies
and lots of unaccessed data. If you keep 3 or 4 or more copies it is not likely
to happen.

You can use 8+2 in version 4, which has the same redundancy level of 3 copies but
the space overhead is only 25%.

If you are using 2 copies now, the space overhead is 50%, 8+2 will give you more
protection with much less space overhead. The only catch with EC introduced in v4
is that you have to have a lot of chunkservers, 12 chunkservers if you use 8+2.

If you are losing sleep over your data currently, you can write a very simple script
that will read your entire filesystem in a loop.
If you have disk space, you can also set your copies to 3 for now and convert to 8+2 later.
It is possible to do up to 8+8 in v4, which has a space overhead of 50% but
it is the equivalent of 9 copies.

The bottom line is that there are many ways to safely protect your data both in version 3
and version 4. If you provide more info about your setup, such as your hardware configuration
and your data size and usage, the community can provide better suggestions.

-- Marco