Date:   Sun, 19 Mar 2017 07:19:01 +0300
From:   Alexey Lyashkov <alexey.lyashkov@...il.com>
To:     Theodore Ts'o <tytso@....edu>
Cc:     Andreas Dilger <adilger@...ger.ca>,
        Artem Blagodarenko <artem.blagodarenko@...il.com>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Yang Sheng <yang.sheng@...el.com>,
        Zhen Liang <liang.zhen@...el.com>,
        Artem Blagodarenko <artem.blagodarenko@...gate.com>
Subject: Re: [PATCH] Add largedir feature

sorry for english..
> 19 марта 2017 г., в 3:39, Theodore Ts'o <tytso@....edu> написал(а):
> 
> On Sat, Mar 18, 2017 at 08:17:55PM +0300, Alexey Lyashkov wrote:
>>> 
>>> That's not true.  We normally only read in one block a time.  If there
>>> is a hash collision, then we may need to insert into the rbtree in a
>>> subsequent block's worth of dentries to make sure we have all of the
>>> directory entries corresponding to a particular hash value.  I think
>>> you misunderstood the code.
>> 
>> As i see it not about hash collisions, but about merging a several
>> blocks into same hash range on up level hash entry.  so if we have a
>> large hash range originally assigned to the single block, all that
>> range will read at memory at single step.  With «aged» directory
>> when hash blocks used already - it’s easy to hit.
> 
> If you look at ext4_htree_fill_tree(), we are only iterating over the
> leaf blocks.  We are using a 31-bit hash, where the low-order bit is
> one if there has been a collision.  In that case, we need to read the
> next block to make sure all of the directory entries which have the
> same 31-bit hash are in the rbtree.
looks we say about same but with different words.
based on dx code, up level hash block have a records - {hash1, block1} {hash2, block1}…
so any records with hash range {hash1, hash2} will live on block1.
right ? 
so question how much {hash1, hash2} distance may be, looks you name it as "hash collision".


> You seem very passionate about this.  Is this a problem you've
> personally seen?  If so, can you give me more details about your use
> case, and how you've been running into this issue?  Instead of just
> arguing about it from a largely theoretical perspective?
> 
It problem was seen with Lustre MDT code  after large number create/unlinks.
but it seen only few times.
Other hits isn’t conformed.



>> Yes, i expect to have some seek penalty. But may testing say it too huge now.
>> directory creation rate started with 80k create/s have dropped to the 20k-30k create/s with hash tree extend to the level 3.
>> Same testing with hard links same create rate dropped slightly.
> 
> So this sounds like it's all about the seek penalty of the _data_
> blocks.  
no.

> If you use hard links the creation rate only dropped a
> little, am I understanding you corretly?
yes and no. hard link create rate dropped a little, but open()+close tests dropped a large.
No writes, no data blocks, just inode allocation.



>  (Sorry, your English is a
> little fracturered so I'm having trouble parsing the meaning out of
> your sentences.)
it’s my bad :(
> 
> So what do you think the creation rate _should_ be?  And where do you
> think the time is going to, if it's not the fact that we have to place
> the data blocks further and further from the directory?  And more
> importantly, what's your proposal for how to "fix" this?
> 
>>> As for the other optimizations --- things like allowing parallel
>>> directory modifications, or being able to shrink empty directory
>>> blocks or shorten the tree are all improvements we can make without
>>> impacting the on-disk format.  So they aren't an argument for halting
>>> the submission of the new on-disk format, no?
>>> 
>> It’s argument about using this feature. Yes, we can land it, but it decrease an expected speed in some cases.
> 
> But there are cases where today, the workload would simply fail with
> ENOSPC when the directory couldn't grow any farther.  So in those
> cases _maybe_ there is something we could do differently that might
> make things faster, but you have yet to convince me that the
> fundamental fault is one that can only be cured by an on-disk format
> change.  (And if you believe this is to be true, please enlighten us
> on how we can make the on-disk format better!)
> 
> Cheers,
> 
> 					- Ted

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