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([2600:4040:530f:b400:d348:ae32:97a6:8a0d]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-9088008e9aasm13982146d6.28.2026.08.04.17.37.54 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 04 Aug 2026 17:37:54 -0700 (PDT) Message-ID: <0255e828-bc7c-46ec-ba72-847e56b44b15@redhat.com> Date: Tue, 4 Aug 2026 20:37:53 -0400 Precedence: bulk X-Mailing-List: dm-devel@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] vdo: add zstd compression support To: June Park , dm-devel@lists.linux.dev References: <20260723161350.1080473-1-june@pythonplayer123.dev> <178a5039-1817-4e13-8937-12eee8b26cc6@redhat.com> <012e81d2-dd6e-4977-b385-d7ac890be2a2@pythonplayer123.dev> From: Matthew Sakai In-Reply-To: <012e81d2-dd6e-4977-b385-d7ac890be2a2@pythonplayer123.dev> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: Nyge8rC6OdJ-VMf9byBEi2Hsbj8NbjDQiysQUZIMUUc_1785890276 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi June, Sorry for the delay in responding. I've been away from the office and I'm currently digging out of the backlog of stuff that's piled up, but I hope to be able to make a real reply to this soon. Matt On 7/27/26 9:43 AM, June Park wrote: > Hello! Thank you for the thoughtful comments. > >> I have to ask, what is the motivation for this change? Do you have a workload that shows some kind of improvement from this proposal? > > My initial testing in QEMU, storing files from the kernel source tree, showed noticeable improvements in compression ratio (around 2.4 to 3.5), even though vdo uses small block sizes. However, I did notice that certain types of data (such as long synthetic streams of repeating bytes) showed far less improvement with zstd, at the expense of more CPU cycles. I had concluded that it could outperform LZ4 in some workloads in terms of compression ratio, for more CPU, a trade-off that could be made on a case-by-case basis. > > The discussion you linked to proposes the ability to swap the algorithm without reformatting. My current implementation tries to reduce breaking changes as much as possible, so I had decided on storing the compression algorithm directly in the volume geometry, instead of for every block. This has the downside of requiring reformatting, but since the focus of vdo is deduplication, I don't think the large amounts of additional machinery needed to support live changes is justified. > > June >