Re: IIs factible to implement full writes of strips to raid using NVRAM memory in LFS?

[<Paul_Koning%Dell.com@localhost>]

> On Aug 18, 2016, at 1:58 PM, Jose Luis Rodriguez Garcia <joseyluis%gmail.com@localhost> wrote:
> 
> On Thu, Aug 18, 2016 at 7:24 PM, Eduardo Horvath <eeh%netbsd.org@localhost> wrote:
>> 
>> LFS writes the metadata at the same time, in the same place as the data.
>> No synchronous writes necessary.
> 
> As I understand LFS needs to do synchronous writes when there is
> metadata operations (directories)/fsync operations involved. Instead
> of writting a full segment (1 MB per default), it writes a "small
> segment". It kills performance in RAID 5/6, because the write isn't a
> full stripe: you have to read all the disks, for calculate the new
> parity 1 write on raid of x disks= x reads + 2 writes (data + parity).

That's not correct.

You're right that partial stripe writes are more expensive, but they are not *that* expensive.  For RAID 5, the cost is two read and two writes: read/write of the data disk, and read/write of the parity disk.  For RAID 6 is it three reads and writes (read/write on both of the P and Q disks).

	paul