Hi all,
I (think of) quite well understand how far and offset work, but I can
not find any data on the precise on-disk layout.
FAR LAYOUT
md(4) states:
"The first copy of all data blocks will be striped across the early part
of all drives in RAID0 fashion, and then the next copy of all blocks
will be striped across a later section of all drives, always ensuring
that all copies of any given block are on different drives"
The "on different drives" part let me wonder _how_ are chunks
distributed. On a 4-disk array, I can imagine some different schemas:
1) A1 A2 A3 A4
.. .. .. ..
A4 A1 A2 A3
2) A1 A2 A3 A4
.. .. .. ..
A2 A1 A4 A3
The first schema is the one depicted by SuSe documentation [1], while
the second is the one described by Wikipedia [2].
Question 1: as the two schema have different reliability
characteristics, which is really used?
OFFSET LAYOUT
md(4) states:
"When 'offset' replicas are chosen, the multiple copies of a given chunk
are laid out on consecutive drives and at consecutive offsets.
Effectively each stripe is duplicated and the copies are offset by one
device."
This means a schema like this:
3) A1 A2 A3 A4
A4 A1 A2 A3
.. .. .. ..
However, this is susceptible to any consecutive two-disk failures. A
schema like
4) A1 A2 A3 A4
A2 A1 A4 A3
would not suffer from this problem (eg: disk 2 & 3 can fail and the
array is still working).
Question 2: apart from simplicity, why the offset layout use the schema
as n.3? I miss something?
Regards.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@xxxxxxxxxx - info@xxxxxxxxxx
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