[linux-lvm] raid & its stripes

[lejeczek]

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 at 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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>


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