On 23/09/17 16:31, lejeczek wrote:
On 18/09/17 17:10, Brassow Jonathan wrote:
On Sep 15, 2017, at 6:59 AM, lejeczek
<peljasz@yahoo.co.uk> wrote:
On 15/09/17 03:20, Brassow Jonathan wrote:
There is definitely a difference here. You have 2
stripes with 5 devices in each stripe. If you were
writing sequentially, you’d be bouncing between the
first 2 devices until they are full, then the next 2,
and so on.
When using the -i argument, you are creating 10
stripes. Writing sequentially causes the writes to go
from one device to the next until all are written and
then starts back at the first. This is a very
different pattern.
I think the result of any benchmark on these two very
different layouts would be significantly different.
brassow
BTW, I swear at one point that if you did not provide
the ‘-i’ it would use all of the devices as a stripe,
such that your two examples would result in the same
thing. I could be wrong though.
that's what I thought I remembered too.
I guess a big question, from user/admin perspective is:
are those two stripes LVM decides on(when no -i) is the
best possible choice LVM makes after some elaborative
determination so the number of stripes(no -i) would,
might vary depending on raid type, phy devices number
and maybe some other factors or, 2 stripes are simply
hard-coded defaults?
If it is a change in behavior, I’m sure it came as the
result of some changes in the RAID handling code from
recent updates and is not due to some uber-intellegent
agent that is trying to figure out the best fit.
brassow
but if confuses, current state of affairs is confusing. To
add to it:
~]# lvcreate --type raid5 -n raid5-0 -l 96%vg caddy-six
/dev/sd{a..f}
Using default stripesize 64.00 KiB.
Logical volume "raid5-0" created.
~]# lvs -a -o +stripes caddy-six
LV VG Attr LSize Pool
Origin Data% Meta% Move Log Cpy%Sync Convert #Str
raid5-0 caddy-six rwi-a-r---
1.75t
0.28 3
[raid5-0_rimage_0] caddy-six Iwi-aor---
894.25g
1
[raid5-0_rimage_0] caddy-six Iwi-aor---
894.25g
1
[raid5-0_rimage_1] caddy-six Iwi-aor---
894.25g
1
[raid5-0_rimage_1] caddy-six Iwi-aor---
894.25g
1
[raid5-0_rimage_2] caddy-six Iwi-aor---
894.25g
1
[raid5-0_rimage_2] caddy-six Iwi-aor---
894.25g
1
[raid5-0_rmeta_0] caddy-six ewi-aor---
4.00m
1
[raid5-0_rmeta_1] caddy-six ewi-aor---
4.00m
1
[raid5-0_rmeta_2] caddy-six ewi-aor---
4.00m
1
VG and LV upon creating was told: use 6 PVs.
How can we rely on what lvcreate does when left to decide
and/or use defaults?
Is above example with raid5 what LVM is suppose to do? Is
it even correct raid5 layout(six phy disks)?
regards.
~]# lvs -a -o +stripes,devices caddy-six
LV VG Attr LSize Pool
Origin Data% Meta% Move Log Cpy%Sync Convert #Str Devices
raid5-0 caddy-six rwi-a-r---
1.75t 2.36
3 raid5-0_rimage_0(0),raid5-0_rimage_1(0),raid5-0_rimage_2(0)
[raid5-0_rimage_0] caddy-six Iwi-aor---
894.25g
1 /dev/sda(1)
[raid5-0_rimage_0] caddy-six Iwi-aor---
894.25g
1 /dev/sdd(0)
[raid5-0_rimage_1] caddy-six Iwi-aor---
894.25g
1 /dev/sdb(1)
[raid5-0_rimage_1] caddy-six Iwi-aor---
894.25g
1 /dev/sde(0)
[raid5-0_rimage_2] caddy-six Iwi-aor---
894.25g
1 /dev/sdc(1)
[raid5-0_rimage_2] caddy-six Iwi-aor---
894.25g
1 /dev/sdf(0)
[raid5-0_rmeta_0] caddy-six ewi-aor---
4.00m
1 /dev/sda(0)
[raid5-0_rmeta_1] caddy-six ewi-aor---
4.00m
1 /dev/sdb(0)
[raid5-0_rmeta_2] caddy-six ewi-aor---
4.00m
1 /dev/sdc(0)
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