linux-ext4 - Re: ext4 scaling limits ?

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Date:   Tue, 21 Mar 2017 22:59:18 +0100
From:   Reindl Harald <h.reindl@...lounge.net>
To:     Andreas Dilger <adilger@...ger.ca>,
        Manish Katiyar <mkatiyar@...il.com>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: ext4 scaling limits ?



Am 21.03.2017 um 22:48 schrieb Andreas Dilger:
> While it is true that e2fsck does not free memory during operation, in
> practice this is not a problem. Even for large filesystems (say 32-48TB)
> it will only use around 8-12GB of RAM so that is very reasonable for a
> server today.

no it's not reasonable even today that your whole physical machine 
exposes it's total RAM to the one of many single virtual machines 
running just a samba server for a 50 TB "datagrave" with a handful of users

in reality it should not be a problem to attach even a 100 TB storage to 
a VM with 1-2 GB

> The rough estimate that I use for e2fsck is 1 byte of RAM per block.
>
> Cheers, Andreas
>
>> On Mar 21, 2017, at 16:07, Manish Katiyar <mkatiyar@...il.com> wrote:
>>
>> Hi,
>>
>> I was looking at e2fsck code to see if there are any limits on running
>> e2fsck on large ext4 filesystems. From the code it looks like all the
>> required metadata while e2fsck is running is only kept in memory and
>> is only flushed to disk when the appropriate changes are corrected.
>> (Except the undo file case).
>> There doesn't seem to be a case/code where we have to periodically
>> flush some tracking metadata while it is running, just because we have
>> too much of incore tracking data and may ran out of memory (looks like
>> code will simply return failure if ext2fs_get_mem() returns failure)
>>
>> Appreciate if someone can confirm that my understanding is correct ?