ext4 stride and stripe-width on top of multiple RAID arrays

eponymous · 04-04-2015, 06:39 AM

I have the following setup:

3x4TB RAID-5 (Hardware RAID) (two data disks URE 1 in 10^15)
5x2TB RAID-5 (Hardware RAID) (four data disks URE 1 in 10^15)

= six data disks in total?

These will be encrypted using LUKS and then collected together into a single Volume Group in which I'll create some logical volumes.

So in creating the Volume Group, how will this setup adjust the --physicalextentsize I need to specify (default is 4MiB)? In this case 4MiB might not be a multiple of the stripe-widths of both arrays so will need to change.

Also, when creating an ext4 filesystem that spans over these two arrays, will the stride and stripe-width options still follow these formulae?:

Code:

stride = chunk_size (bytes) / 4096

stripe_width = (number of data disks) * (chunk_size in bytes) / 4096  
stripe_width = 6 * (chunk_size in bytes) / 4096

I'm assuming there are six data disks but I'm not sure this is correct since they're of different sizes in different arrays. i.e. four disks are 2TB and two disks are 4TB.

Does anyone know if I've missed something here?

Note: I used this page to aid me with calculations.

Thanks.

jheengut · 04-07-2015, 02:38 AM

Quote:

Originally Posted by eponymous

I have the following setup:

3x4TB RAID-5 (Hardware RAID) (two data disks URE 1 in 10^15)
5x2TB RAID-5 (Hardware RAID) (four data disks URE 1 in 10^15)

= six data disks in total?

I'm assuming there are six data disks but I'm not sure this is correct since they're of different sizes in different arrays. i.e. four disks are 2TB and two disks are 4TB.

Thanks.

So you have 8TB + 8TB of RAID5 data in all.

jheengut · 04-07-2015, 02:49 AM

Quote:

Originally Posted by eponymous

So in creating the Volume Group, how will this setup adjust the --physicalextentsize I need to specify (default is 4MiB)? In this case 4MiB might not be a multiple of the stripe-widths of both arrays so will need to change.

Thanks.

from man::
s, --physicalextentsize PhysicalExtentSize[bBsSkKmMgGtTpPeE] Sets the physical extent size on physical volumes of this volume group. The default is 4 MiB and it must be at least 1 KiB and a power of 2.
If the volume group metadata uses lvm1 format, extents can vary in size from 8KiB to 16GiB and there is a limit of 65534 extents in each logical volume. The default of 4 MiB leads to a maximum logical volume size of around 256GiB.
If the volume group metadata uses lvm2 format those restrictions do not apply, but having a large number of extents will slow down the tools but have no impact on I/O performance to the logical volume. The smallest PE is 1KiB

so why the need to change PhysicalExtentSize

If in doubt use a bigger PhysicalExtentSize