On 22/07/2013 08:03, Charles 'Boyo wrote:
Counting on the kernel's cache, it appears I will be best served
purchasing write-optimized SSDs?
Can you share any information on the SSD you are using, is it PCIe
connected?
We are on a standard SAS bus so any SSD going to 500MB/s and being
stable on the long run (we use 60G Intel 520), you do not need a lot of
space for the journal (5G per drive is far enough on commodity hardware).
Another question, since the intention of this storage cluster is
relatively cheap storage on commodity hardware, what's the balance
between cheap SSDs and reliability since journal failure might result
in data loss or will such an event just 'down' the affected OSDs?
A journal failure will fail your OSDs (from what I've understood, you'll
have to rebuild them). But SSDs are very deterministic, so monitor them :
# smartctl -A /dev/sdd
[..]
ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE
UPDATED WHEN_FAILED RAW_VALUE
[..]
232 Available_Reservd_Space 0x0033 100 100 010 Pre-fail
Always - 0
233 Media_Wearout_Indicator 0x0032 093 093 000 Old_age
Always - 0
And don't put too many OSDs on one SSD (I set a rule to not go over 4
for 1).
On a similar note, I am using XFS on the OSDs which also journals,
does this affect performance in any way?
You want this journal for consistency ;) I don't know exactly the
impact, but since we use spinning drives, the most important factor is
that ceph, with a journal on SSD, does a lot of sequential writes,
avoiding most seeks.
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