Why it takes 6% of SCM to record the metadata?


cheneydeng@...
 

Hi DAOS,

I know daos use the SCM to store metadata, I can't find a wiki to describe the details about what and how we stored in the metadata, I guess there must be something like block locations(mapping LBA to PBA) and allocation bookeepers, but I don't understand why it takes so much space? Typically it only takes 0.1% of the SSD space to record metadata in the normal SSD FTL, what's the difference?

And I'm also wondering if the community has some directions on how to use less SCM?

DJ.


Zhen, Liang
 

Hi DJ,

 

DAOS is using both SCM and SSD for application dataset,  so the 6% is not just for system metadata of DAOS, it is also for user/application metadata and small data. For example, keys and small values(<4k) of KV store, inode and small files of POSIX container, …

 

Regards

Liang

 

From: daos@daos.groups.io <daos@daos.groups.io> on behalf of cheneydeng via groups.io <cheneydeng@...>
Date: Monday, October 25, 2021 at 3:00 PM
To: daos@daos.groups.io <daos@daos.groups.io>
Subject: [daos] Why it takes 6% of SCM to record the metadata?

Hi DAOS,

I know daos use the SCM to store metadata, I can't find a wiki to describe the details about what and how we stored in the metadata, I guess there must be something like block locations(mapping LBA to PBA) and allocation bookeepers, but I don't understand why it takes so much space? Typically it only takes 0.1% of the SSD space to record metadata in the normal SSD FTL, what's the difference?

And I'm also wondering if the community has some directions on how to use less SCM?

DJ.


cheneydeng@...
 

Hi Liang,

Thanks for your reply, I'll consider the 6% as a recommendation for using DAOS.

While I have a further question is that, I see the core data struct of metadata in SCM is b+tree, which is disk-friendly, I'm wondering if I would like to not to use SCM but to store all the metadata on the disk, what do you think the complexity will be?

DJ.  


Lombardi, Johann
 

For the record, we also have a python script to estimate the amount of SCM vs NVMe required for a given middleware/object distribution. See https://github.com/daos-stack/daos/tree/master/src/vos/storage_estimator for more information.

 

Cheers,

Johann

 

From: <daos@daos.groups.io> on behalf of "Zhen, Liang" <liang.zhen@...>
Reply-To: "daos@daos.groups.io" <daos@daos.groups.io>
Date: Tuesday 26 October 2021 at 09:14
To: "daos@daos.groups.io" <daos@daos.groups.io>
Subject: Re: [daos] Why it takes 6% of SCM to record the metadata?

 

Hi DJ,

 

DAOS is using both SCM and SSD for application dataset,  so the 6% is not just for system metadata of DAOS, it is also for user/application metadata and small data. For example, keys and small values(<4k) of KV store, inode and small files of POSIX container, …

 

Regards

Liang

 

From: daos@daos.groups.io <daos@daos.groups.io> on behalf of cheneydeng via groups.io <cheneydeng@...>
Date: Monday, October 25, 2021 at 3:00 PM
To: daos@daos.groups.io <daos@daos.groups.io>
Subject: [daos] Why it takes 6% of SCM to record the metadata?

Hi DAOS,

I know daos use the SCM to store metadata, I can't find a wiki to describe the details about what and how we stored in the metadata, I guess there must be something like block locations(mapping LBA to PBA) and allocation bookeepers, but I don't understand why it takes so much space? Typically it only takes 0.1% of the SSD space to record metadata in the normal SSD FTL, what's the difference?

And I'm also wondering if the community has some directions on how to use less SCM?

DJ.

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