Is Kernel Cache exists for Physical Raw Device Mapping ?

[MTBShoichi]

Hello,

I am using a disk of SAN Storage attached to Guest OS as "Physical Raw Device Mapping", and mounted at /mnt/disk1.

I'd lke to take backup of /mnt/disk1 with "SAN Storage FlashCopy function", that copies entire of disk within SAN Storage Box bypassing Linux operating system and VMware kernel.

Abstract of backup script is as following.

Step1: sync ' Flush cache of Linux Operatiog System

Step2: umount /mnt/disk1

Step3: ExecFlashCopy ' Execute SAN Storage FlashCopy function

Step4: mount /dev/sde /mnt/disk1

But, if VMware kernel has cache for "Physical Raw Device Mapping Device", "Flush VMware Cache" is required at Step 2.5.

Do you know VMware cache exists for P"hysical Raw Device Mapping Device", and how can I flush it.

Thanks.

[AntonVZhbankov]

VMware ESXi does not have any caches for disk IO to provide 100% data consistency - the only cache can be in guest OS.

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MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert

http://blog.vadmin.ru

[MTBShoichi]

Thank you very much for replying my question, Anton.

I am little confused because you wrote, "VMware ESXi does not have any

caches for disk IO to provide 100% data consistency - the only cache

can be in guest OS.".

When I create datastore from SAN Disk, and assign virtual disk from it,

I guess VMware has cache for it, because datastore is a device with

filesystem (VMFS) of VMware (linux).

I think you wrote "the only cache can be in guest OS" for Raw Device

Mapping case only, but I want to make it sure.

Is it correct that "Is VMware has cache for Physical Raw Device Mapping ?

And do you know that s VMware has cache for Virtual Raw Device Mapping ?

Thank you very much.

[AntonVZhbankov · Accepted solution]

Your guess is wrong. ESXi is not linux and does not have cache for VMFS.

ESXi does not cache any VM IO, guest OS receives IO acknowledgment when disk controller acknowledges IO completion.

ESXi does not have any cache for RDM, no matter physical or virtual.

---

MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert

http://blog.vadmin.ru

[MTBShoichi]

Thank you very much for replying, Anton.

I thought VMware is working under linux by some reasons, i.e. change

virtual console by Alt+F1, or enable ssh by /etc/inetd.conf. ...,

but I knew I have wrong assumption.

Thank you very much, and it is a very helpfull answer.

[AntonVZhbankov]

Your impression is wrong. What you see is Service Console, a kind of privileged virtual machine used to perform some infrastructure and management tasks, but it is working under VMware hypervisor.

Service console in ESX is RHEL-based, yes. It is special but still virtual machine.

So answer is still no - no caching for VMFS.

---

MCSA, MCTS Hyper-V, VCP 3/4, VMware vExpert

http://blog.vadmin.ru