Is there file size limit to ZFS ARC

[strikeraj]

Hi
I am not using TrueNAS but there are a lot of experts here on ZFS so I would try my luck.

Question:
I am running optimization in a W10 guest VM that reads the same 16GiB dataset repeatedly for each loop. However, after running that a couple days, it is still being read from the pool instead of from ARC. Does anyone have a clue what may be the problem? Is the file too big? Or is there limitation on what ARC on host can do about files in guest VM?

Here is my setup.

Dell R630 with 384GiB ram
ZFS RaidZ1 with 3x 512GB SATA SSD
Proxmox PVE
zfs-2.1.12-pve1
zfs-kmod-2.1.12-pve1
ARC min = 128 GiB
ARC max = 256 GiB

Thanks in advance!

 

[Patrick M. Hausen]

ARC is per block, not per file. Possibly the data is not read frequently enough and gets evicted for "hotter" blocks?

 

[strikeraj]

ARC is per block, not per file. Possibly the data is not read frequently enough and gets evicted for "hotter" blocks?

That should not be the case, as i am just test running the server with that one code.
Also, the ARC usage is not even close to being full (arc_summary only showing 3GB)

 

[sfatula]

If your only use is the one VM, then, the arc will not be full of course as nothing (minimal stuff) is being read otherwise. So, arc not being full would be expected.

What does thus command show (replacing yourdatasetname with the dataset name of the windows dataset of course):

zfs get primarycache,secondarycache yourdatasetname

Did you change any other zfs tunable and if so what?

How often does your VM loop run?

 

[Davvo]

Please post arc_summary output inside [CODE][/CODE] as well.
Back when there was the ARC eviction bug in 13-U4 I used the following tunable: vfs.zfs.arc.meta_prune set to 0, type SYSCTL; I don't know if it might help.

 

[sfatula]

Is the file itself 16GB in size, or, is the Windows file smaller and therefore might be cached by Windows (which happens before zfs)?

How do you know it's reading from the pool?

There are still some eviction bugs around, one that comes to mind:

Data throughput causing apparent (directory) metadata eviction with metadata_size << arc_meta_min · Issue #10508 · openzfs/zfs

System information Type Version/Name Distribution Name Proxmox VE (some configs below were repeated on Ubuntu 20.04 with the same result) Distribution Version 6.2 and 6.4-1 Linux Kernel (tested acr...

github.com

 

[strikeraj]

Please post arc_summary output inside [CODE][/CODE] as well.
Back when there was the ARC eviction bug in 13-U4 I used the following tunable: vfs.zfs.arc.meta_prune set to 0, type SYSCTL; I don't know if it might help.

Code:

ARC status:                                                      HEALTHY
        Memory throttle count:                                         0

ARC size (current):                                    43.8 %  112.2 GiB
        Target size (adaptive):                        50.0 %  128.0 GiB
        Min size (hard limit):                         50.0 %  128.0 GiB
        Max size (high water):                            2:1  256.0 GiB
        Most Frequently Used (MFU) cache size:         77.0 %   85.9 GiB
        Most Recently Used (MRU) cache size:           23.0 %   25.6 GiB
        Metadata cache size (hard limit):              75.0 %  192.0 GiB
        Metadata cache size (current):                  0.5 %  885.3 MiB
        Dnode cache size (hard limit):                 10.0 %   19.2 GiB
        Dnode cache size (current):                   < 0.1 %    2.5 MiB

ARC hash breakdown:
        Elements max:                                               2.2M
        Elements current:                              98.9 %       2.2M
        Collisions:                                               311.2k
        Chain max:                                                     3
        Chains:                                                    35.8k

ARC misc:
        Deleted:                                                      14
        Mutex misses:                                                  0
        Eviction skips:                                                1
        Eviction skips due to L2 writes:                               0
        L2 cached evictions:                                     0 Bytes
        L2 eligible evictions:                                 247.0 KiB
        L2 eligible MFU evictions:                      0.0 %    0 Bytes
        L2 eligible MRU evictions:                    100.0 %  247.0 KiB
        L2 ineligible evictions:                                 4.0 KiB

ARC total accesses (hits + misses):                                 4.3G
        Cache hit ratio:                              100.0 %       4.3G
        Cache miss ratio:                             < 0.1 %      35.2k
        Actual hit ratio (MFU + MRU hits):            100.0 %       4.3G
        Data demand efficiency:                       100.0 %       2.2G
        Data prefetch efficiency:                       1.9 %      34.1k

Cache hits by cache type:
        Most frequently used (MFU):                    99.6 %       4.3G
        Most recently used (MRU):                       0.4 %      18.2M
        Most frequently used (MFU) ghost:               0.0 %          0
        Most recently used (MRU) ghost:                 0.0 %          0
        Anonymously used:                             < 0.1 %       3.4k

Cache hits by data type:
        Demand data:                                   51.4 %       2.2G
        Prefetch data:                                < 0.1 %        633
        Demand metadata:                               48.6 %       2.1G
        Prefetch metadata:                            < 0.1 %     878.8k

Cache misses by data type:
        Demand data:                                    2.9 %       1.0k
        Prefetch data:                                 95.0 %      33.5k
        Demand metadata:                                1.8 %        635
        Prefetch metadata:                              0.3 %        110

DMU prefetch efficiency:                                          544.3M
        Hit ratio:                                     32.8 %     178.3M
        Miss ratio:                                    67.2 %     366.0M

L2ARC not detected, skipping section

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ZIL committed transactions:                                        10.2M
        Commit requests:                                          575.7k
        Flushes to stable storage:                                575.7k
        Transactions to SLOG storage pool:            0 Bytes          0
        Transactions to non-SLOG storage pool:       36.6 GiB     773.9k

 

[strikeraj]

Is the file itself 16GB in size, or, is the Windows file smaller and therefore might be cached by Windows (which happens before zfs)?

How do you know it's reading from the pool?

There are still some eviction bugs around, one that comes to mind:

Data throughput causing apparent (directory) metadata eviction with metadata_size << arc_meta_min · Issue #10508 · openzfs/zfs

System information Type Version/Name Distribution Name Proxmox VE (some configs below were repeated on Ubuntu 20.04 with the same result) Distribution Version 6.2 and 6.4-1 Linux Kernel (tested acr...

github.com

It is 16GB in size.
From Windoes task manager, I can see it is reading from disk. In addition, in arc_summary, the ARC size is smaller than that file size.

 

[strikeraj]

If your only use is the one VM, then, the arc will not be full of course as nothing (minimal stuff) is being read otherwise. So, arc not being full would be expected.

What does thus command show (replacing yourdatasetname with the dataset name of the windows dataset of course):

zfs get primarycache,secondarycache yourdatasetname

Did you change any other zfs tunable and if so what?

How often does your VM loop run?

the part that I am confused is that the arc total assess is smaller than that one file I am looping in windows, which loops about 200 times in 24 hours.
In this set up, I am using default zfs setup, outside of changing the min and max arc size.

The command gives this output

Code:

NAME       PROPERTY        VALUE           SOURCE
sata_pool  primarycache    all             default
sata_pool  secondarycache  all             default

 

[sfatula]

It is 16GB in size.
From Windoes task manager, I can see it is reading from disk. In addition, in arc_summary, the ARC size is smaller than that file size.

From the point of view of Windows, it would be reading from disk even if it were all in the arc. How much cache does Windows use? Perhaps some is in Windows cache, and the rest in the arc?

You have a 100% arc hit ratio, which would indicate to me it is working. The arc is also way bigger than the 16GB file, it's over 100GB. All that makes sense to me. You sure it isn't just because you see Windows saying disk IO, because coming from the arc, Windows does not know that in a VM. It should be just asking for IO from the "disk", the disk being zfs which then gets it from the arc.

My understanding of arc total access is 4.3G means 4.3 billion hits + misses, which again, sounds reasonable to me.

 

[Davvo]

Try running cmdwatch -n 2 zpool iostat -v, see if something moves when reading the file.

 

[Arwen]

Also note that if you have access time enabled on the MS-Windows side, MS-Windows would be writing to the zVol to update last access. I am not sure if MS-Windows can disable access time on files.

ZFS zVols don't have an access time variable. Nor do I see anything that might help on the ZFS zVol side.

All that is a guess, partly because this is writing, and also because my knowledge of MS-Windows file systems is limited.

 

[strikeraj]

Try running cmdwatch -n 2 zpool iostat -v, see if something moves when reading the file.

I got command not found. Is there something I need to install to use ?

 

[jgreco]

ARC is per block, not per file.

It also does not favor large sequential files, which are assumed to be more easily pulled from the pool on demand, as sequential disk access is very fast. ARC tends to favor trying to reduce seeks.

 

[Davvo]

I got command not found. Is there something I need to install to use ?

My bad, I believed you were running CORE; for SCALE run watch -n 2 zpool iostat -v.

 

[winnielinnie]

My bad, I believed you were running CORE

I can't tell you how many times I pull my hair out and shake my fist in the air when I have to adapt commands and syntax, back and forth, between FreeBSD and Linux.

tar?

sed?

watch?

And so on...

It's mostly the same... except it's not.

 

[Davvo]

I can't tell you how many times I pull my hair out and shake my fist in the air when I have to adapt commands and syntax, back and forth, between FreeBSD and Linux.

tar?

sed?

watch?

And so on...

It's mostly the same... except it's not.

That's why engineers are bald.

 

[Arwen]

But, but that is what SHELL aliases are for;

alias watch=cmdwatch
alias tar=gtar

And if the command line parameters are too different, then setup a simple SHELL script to handle the differences.

Both, (alias or script), only have to be done once, as long as you keep a master location. If you want, you can even wrap the aliases in IF/THEN/ELSE so that they are not run from the wrong OS.

 

[winnielinnie]

Or we can sit down Richard Stallman, Linus Torvalds, and developers from all projects to enjoy a five-star dinner together, so that they can peacefully discuss and settle on a single universal standard. I'm sure they'll all agree. We'll even pay for their food.

 

[jgreco]

But, but that is what SHELL aliases are for;

But we were just hearing in another thread about how it was bad that iX developers should have included a PATH for stuff in /usr/sbin? Surely you are not suggesting that dotfiles are a user's responsibility?!?!?!?! (is there no sarcasm tag?!?!?!?!)