linux-kernel - Re: [PATCH 0/1] cover-letter/lz4: Implement lz4 with dynamic offset length.

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Date:   Mon, 02 Apr 2018 11:33:54 +0530
From:   Maninder Singh <maninder1.s@...sung.com>
To:     "sergey.senozhatsky.work@...il.com" 
        <sergey.senozhatsky.work@...il.com>
CC:     "herbert@...dor.apana.org.au" <herbert@...dor.apana.org.au>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "minchan@...nel.org" <minchan@...nel.org>,
        "ngupta@...are.org" <ngupta@...are.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "anton@...msg.org" <anton@...msg.org>,
        "ccross@...roid.com" <ccross@...roid.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "colin.king@...onical.com" <colin.king@...onical.com>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        PANKAJ MISHRA <pankaj.m@...sung.com>,
        AMIT SAHRAWAT <a.sahrawat@...sung.com>,
        Vaneet Narang <v.narang@...sung.com>
Subject: Re: [PATCH 0/1] cover-letter/lz4: Implement lz4 with dynamic offset
 length.


 Hello,

>> (Added cover letter to avoid much text in patch description)
>> 
>> LZ4 specification defines 2 byte offset length for 64 KB data.
>> But in case of ZRAM we compress data per page and in most of
>> architecture PAGE_SIZE is 4KB. So we can decide offset length based
>> on actual offset value. For this we can reserve 1 bit to decide offset
>> length (1 byte or 2 byte). 2 byte required only if ofsset is greater than 127,
>> else 1 byte is enough.
> 
>So what happens if I compress the data on a system with no dyn
>offset and then send it over the network to a machine which has
>dyn offset? Or, say, I have a USB stick with a compression enabled
>FS, store files on a dyn offset enabled PC and then mount that USB
>stick on a machine with no dyn offset support. And vice versa.


lz4_dyn is not an extension of LZ4 so there is no backward compatibility.
Consider this as a different algorithm adapted from LZ4 for better compression ratio.

Thanks
Maninder Singh