Update on issue. I came across the "mismatch_cnt" stat after init resync or just common check/scrub. mismatch_cnt suppose to be 0. I had counter reaching millions. I haven't found definitive answer whether "mismatch_cnt" bad, and should be ignored. Allegedly high mismatch_cnt count suggest HW or SW issues. I run SMART, no system corruption. I have tried to run "repair". Repair 400-500MBs, and mismatch_cnt is now 0. Initial resync started at 500MBs, 20 min later dropped to 200 MBs. 3 days later speed was at 120MBs. I will try some real tests to see if array is still fast in real life as well. Hajes -------- Original Message -------- On 08/03/2025 18:59, David Hajes <d.hajes29a@xxxxx> wrote: > > In case someone wonders in future why SW RAID5 or 10 is slow. > > Unless, two or more process do not write in parallel - ARRAY SPEED WILL ALWAYS BE SINGLE DRIVE LIMIT. > > Basically, any single user operations of storing data on the array will became as writing to single drive. > > In case of modern SATA HDDs, it would be 120-220MBs > > Only the HW RAID controllers are allegedly capable to write on more than one drive parallel thus achiving the logical/envisioned/intuitive speed. > > based on theory where at least two chunks are written at once to the two different drives thus doubling the writing speed as confussingly written in all RAID wikis. > > > -------- Original Message -------- > On 07/03/2025 21:47, Roman Mamedov <rm@xxxxxxxxxxx> wrote: > > > On Fri, 7 Mar 2025 14:42:24 -0600 > > Roger Heflin <rogerheflin@xxxxxxxxx> wrote: > > > > > I put an external bitmap on a raid1 SSD and that seemed to speed up my > > > writes. I am not sure if external bitmaps will continue to be > > > supported as I have seen notes that I don't exactly understand for > > > external bitmaps, and I have to reapply the external bitmap on each > > > reboot for my arrays which has some data loss risks in a crash case > > > with a dirty bitmap. > > > > > > This is the command I used to set it up. > > > mdadm --grow --force --bitmap=/mdraid-bitmaps/md15-bitmap.img /dev/md15 > > > > In this case the result cited seems to have shown the bitmap is not the issue. > > > > I remember seeing patches or talks to remove external bitmap support, too. > > > > In my experience the internal bitmap with a large enough chunk size does not > > slow down the write speed that much. Try a chunk size of 256M. Not sure how > > high it's worth going before the benefits diminish. > > > > -- > > With respect, > > Roman > >
