Re: measuring swap partition speed

A bit weird that a simple dd is so slow.

Just a quick thought.
Are your partitions aligned properly?
Is other IO going on on the same time?
What does gstat say about %util, queue length and all the other stats while running the dd? Or "iostat -x -d 1".

Could you try if another disk has the same issues?

How is your disk connected? USB-to-SATA-adapter? Any output of dmesg?

Regards,
Ronald.

 
Van: void <void@f-m.fm>
Datum: donderdag, 21 december 2023 16:50
Aan: freebsd-fs@freebsd.org
Onderwerp: Re: measuring swap partition speed
> 
> On Wed, Dec 20, 2023 at 07:48:14PM -0800, Mark Millard wrote:
> 
> ># swapoff /dev/label/growfs_swap
> ># dd if=/dev/urandom of=/dev/da0s2b bs=8k count=250000 conv=sync status=progress
> >^C478830592 bytes (479 MB, 457 MiB) transferred 22.001s, 22 MB/s
> >60557+0 records in
> >60556+0 records out
> >496074752 bytes transferred in 22.790754 secs (21766491 bytes/sec)
> 
> 22MB/s is usable, I think. In my context, I'd be satisfied with that.
> My context differs from yours slightly in that yours is SSD and mine
> is spinning rust.
> 
> This is unusable:
> # dd if=/dev/urandom of=/dev/da0p4 bs=8k count=250000 conv=sync status=progress
> ^C11862016 bytes (12 MB, 11 MiB) transferred 40.063s, 296 kB/s   
> 
> because it's way too slow. Swap never gets fully reclaimed,
> thrashing happens, loads of other followon effects happen.
> The same partition formatted as ufs reports 113 MB/s. Multiple swap partitions
> have been tested, then converted to ufs. Results are the same.
> 
> There are no reported errors in smartctl. Long smartctl tests run monthly.
> 
> 5 Reallocated_Sector_Ct   PO--CK   100   100   050    -    0
> 9 Power_On_Hours          -O--CK   001   001   000    -    48992
> 196 Reallocated_Event_Count -O--CK   100   100   000    -    0
> 197 Current_Pending_Sector  -O--CK   100   100   000    -    0
> 198 Offline_Uncorrectable   ----CK   100   100   000    -    0
> 
> I can't find any hardware problem here. Possible workarounds, bearing in mind I'm not versant in C so it's not like I can fix this myself in code:
> 
> 1. swap as swapfile and not partition [a]
> 2. swap as nfs [b]
> 3. swapoff & swapon script running every minute [c]
> 4. just turn all swap off and reboot after crashing (undesirable)
> 5. use another OS that doesn't have this problem
> 
> [a] not tried yet, and i hope it works. Legacy info suggests swap as partition is usually
>      faster than filesystem-based swap. But the reverse might be the case here.
> 
> [b] also not tried. This, I imagine, would be filesystem only (I'm unsure a zfs volume can
>      be exported to look like a mountable partition to the client)
> 
> [c] https://github.com/Freaky/swapflush.git - usually works but maybe i need to run it every      minute instead of every five mins. For testing, this script was disabled.
> 
> Any additional suggestions on how to overcome this problem gratefully received.
> 
> -- 
>  
> 
> 
>