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PVS: Cache in Device RAM with Overflow on Hard Disk - Questions about sizing


Teun Visser1709159476

Question

Dear Citrix community,

 

At one of our customers we're creating a new VDI deployment based on Windows 10 (Build 1709).

We're facing some issues with hanging / freezing VDI's. I think to know where the issue lies, I just need some explaination on the subject. Hoping you guys can help me out.

 

Setup as follows:

- Hypervisor: VMWare vSphere / ESXi 6.5

- Desktop Provisioning Method: Citrix Provisioning Services (via vDisk streaming)

- Provisioning Type: Non persistant pooled VDI's (Allocation Type: Random)

- Desktop OS: Windows 10

- Assigned amount of memory to VM: 6 GB (6144 MB)

- Assigned amount of Virtual CPU's to VM: 3

- Write Cache disk size: 6 GB (6144 MB) (disk has been mapped to drive letter D:\)

 

vDisk Specifications:

- Cache in device RAM with overflow on hard disk

- Maximum RAM size (MBs): 2048

 

Page file config has been set in the registry as follows while creating the vDisk:

[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management]
Key Name: "PagingFiles"

Type: REG_MULTI_SZ

Data: D:\pagefile.sys 1024 6144

According to the following article provided by Microsoft: https://support.microsoft.com/nl-nl/help/2860880/how-to-determine-the-appropriate-page-file-size-for-64-bit-versions-of

 

Now, the issue we're facing:

Whenever a VDI boots up, everythings fine, resource usage seems to be allright. A user can login, performance is excellent.

But:... depending on the workload the user puts on the machine it'll freeze up at some point during their login time.

Monitoring the VM shows the following:

When a user logs in memory usage is at 3,8 GB (according to Windows). Of which 1.9 GB is nonpaged pooled. (https://imgur.com/17Z5y2z)

So thats 1.9 GB for the OS and user processes, and 1.9GB claimed by PVS. (should actually be 2048MB claimed, but Windows shows a little less space claimed?)

Then, when I examine the write cache disk, it shows the following: pagefile is up to 1050MB (1.05GB), and the differencing disk is at 4 MB. (https://imgur.com/a/ibzVPZs)

 

Now at this point, whenever I write about 500MB data to the VM, i'll freeze. Leaving the VM non-responsive.

When I up the size of the write-cache disk from 6 to 10 GB it will allow me to write about 1 GB to the VM, but then; it freezes.

So i'm guessing i'm either going to have to up the size of the write-cache disk, or going to have to change something else in above config.

 

Would like to have some assistance / guidance on the sizing made above.

 

Looking forward to your reply's!

 

Greetings,

 

Teun

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8 answers to this question

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Hi Tuen. :)

 

I would suggest setting the page file to a static size, and increase the size of your cache disk...to something like 20 GB.

 

Based on your settings, your page file could completely consume the cache disk by itself, which would leave no room for any other files being written to the C drive.  And....while Windows 10 might be the best DOS yet, it still doesn't like its C drive filling up.

 

Good luck to you. :)

 

-S

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14 hours ago, Stanley Schwartz said:

Hi Tuen. :)

 

I would suggest setting the page file to a static size, and increase the size of your cache disk...to something like 20 GB.

 

Based on your settings, your page file could completely consume the cache disk by itself, which would leave no room for any other files being written to the C drive.  And....while Windows 10 might be the best DOS yet, it still doesn't like its C drive filling up.

 

Good luck to you. :)

 

-S

Hi Stanley,

 

What static size for the page file would you recommend in above outlined setup? Static to 4 GB? And why 4 GB?

Would the page file automatically be set to 4 GB in size on the write cache disk?

And why would 20 GB be a good size for the write cache disk? Why not, for example 10 or 15 GB?

 

I know that when setting the page file to a size of 4 GB static and increasing the write cache disk size would probably mean the freezing of VDI's stops. But i'm looking for reasoning behind decisions to be made.

 

Looking forward to your reply!

 

 

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So sizing a cache drive is very hard to come up with a generic formula that everyone can use since workloads and applications can vary so much.  Please read https://www.citrix.com/blogs/2015/01/19/size-matters-pvs-ram-cache-overflow-sizing/  this will give you some idea of how the cache works and how vdisk maintenance can affect the size.    One trick you can always try to get an idea of how big your cache drive will be is to use server side cache and see how large a typical workload grows the cache on the server.  The server side cache will have the page file and all the writes from the target so this will give you an idea, but again depending on workload and usage the cache could be larger or smaller, I would always side on being larger.   RAM cache with overflow remember the page file will be redirected to the cache drive, normally, so it will not be in the cache file. 

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On 7/28/2018 at 4:54 AM, Teun Visser1709159476 said:

Hi Stanley,

 

What static size for the page file would you recommend in above outlined setup? Static to 4 GB? And why 4 GB?

Would the page file automatically be set to 4 GB in size on the write cache disk?

And why would 20 GB be a good size for the write cache disk? Why not, for example 10 or 15 GB?

 

I know that when setting the page file to a size of 4 GB static and increasing the write cache disk size would probably mean the freezing of VDI's stops. But i'm looking for reasoning behind decisions to be made.

 

Looking forward to your reply!

 

 

I would recommend at least the amount of RAM you have, or a general rule of thumb of 1.5 times the amount of RAM you have.  There ARE better ways to actually figure out how much pagefile a particular machine needs, but since mine are non-persistent, general use, I use the 1.5 times RAM for mine.

 

20 GB cache disk is just what I use.  Windows doesn't like any drive filling up, but it REALLY doesn't like its system drive filling up.  It really sounds like that's what you're experiencing.  Windows write LOTS of crap just idling.  it's always writing event logs and temp files, without the user doing ANYthing.  So when a user starts running applications that create more temp files, or browsing and downloading stuff from the internet, the drive can start filling up quick.

 

So if you make a 20 GB cache disk, and you set your pagefile to static 8 or 10 GB, that now leaves your cache space at 12 or 10 GB, respectively.  Which I think is a much safer size....

 

Curious to hear how it works out for you!

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Yeah, this is late to the conversation but I usually just disable the page file altogether.  Typical VDIs sizing is 2vCPU and 4GB of RAM with 512MB RAM cache with overflow (spinning disks.) Cache disk is 20GB thin provisioned.  Once the desktop is loaded, it is surprisingly snappy.  This works for us particularly well as only a subset of apps run on the VDI with the rest delivered via XenApp.     

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On 1/7/2019 at 3:59 PM, Randy Mysliwiec1709160374 said:

Yeah, this is late to the conversation but I usually just disable the page file altogether.  Typical VDIs sizing is 2vCPU and 4GB of RAM with 512GB RAM cache with overflow (spinning disks.) Cache disk is 20GB thin provisioned.  Once the desktop is loaded, it is surprisingly snappy.  This works for us particularly well as only a subset of apps run on the VDI with the rest delivered via XenApp.     

What OS version...Win 7 or win 10?  4GB seems under provisioned for Win 10.

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I am jumping on this topic after two years, thought it is better to continue here that creating a new track entirely.

 

All had been said, analyzed and suggested as per "page file" and "cache disk size", but is there any rule of thumb / suggestion / recommendation as to the ratio/percentage of the actual RAM to be configured as RAM cache "RAM buffer".

 

 We have been using "Cache on device RAM with overflow on hard disk" option for quite a while, but now contemplating increasing the setting for performance in some intense graphic applications, without overkilling the published devices.

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