From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from gate001.proxmox.com (gate001.proxmox.com [IPv6:2a0f:8001:1:32::40]) by lore.proxmox.com (Postfix) with ESMTPS id 68A3E1FF0C0 for ; Wed, 04 Nov 2026 05:05:28 +0100 (CET) Received: from gate001.proxmox.com (localhost.localdomain [127.0.0.1]) by gate001.proxmox.com (Proxmox) with ESMTP id C9926213D3; Wed, 04 Nov 2026 05:05:21 +0100 (CET) From: Kefu Chai To: pve-devel@lists.proxmox.com Subject: [PATCH pve-doc] pveceph: advise against using FastEC in production Date: Wed, 4 Nov 2026 12:05:01 +0800 Message-ID: <20261104040501.2476504-1-k.chai@proxmox.com> X-Mailer: git-send-email 2.47.3 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Bm-Milter-Handled: 55990f41-d878-4baa-be0a-ee34c49e34d2 X-Bm-Transport-Timestamp: 1793765105117 X-SPAM-LEVEL: Spam detection results: 0 AWL 0.252 Adjusted score from AWL reputation of From: address DMARC_MISSING 0.1 Missing DMARC policy KAM_DMARC_STATUS 0.01 Test Rule for DKIM or SPF Failure with Strict Alignment (newer systems) RCVD_IN_DNSWL_MED -2.3 Sender listed at https://www.dnswl.org/, medium trust SPF_HELO_NONE 0.001 SPF: HELO does not publish an SPF Record SPF_PASS -0.001 SPF: sender matches SPF record Message-ID-Hash: 7FAIFYMIGXVMPUCW5B4ZLBJQW3EOW7RD X-Message-ID-Hash: 7FAIFYMIGXVMPUCW5B4ZLBJQW3EOW7RD X-MailFrom: k.chai@proxmox.com X-Mailman-Rule-Misses: dmarc-mitigation; no-senders; approved; loop; banned-address; emergency; member-moderation; nonmember-moderation; administrivia; implicit-dest; max-recipients; max-size; news-moderation; no-subject; digests; suspicious-header X-Mailman-Version: 3.3.10 Precedence: list List-Id: Proxmox VE development discussion List-Help: List-Owner: List-Post: List-Subscribe: List-Unsubscribe: The FastEC section reads as an endorsement and describes enabling the flag on a populated pool as a change to pool metadata only. That does not match what users have seen. Since Tentacle was released, FastEC has hit several bugs [1][2][3][4][5], mostly around truncate, discard and object size handling, and mostly on pools that already held data when the flag was set. The symptoms range from bogus scrub errors, over 'pg repair' making objects unfound, to OSDs crash-looping on their first reconstruct read and taking the pool inactive. Some of the fixes do not repair damage that was already done, and more fixes for peering, recovery and the extent cache are still open upstream. The text avoids version numbers, as they would be stale soon. [1] https://tracker.ceph.com/issues/71642 [2] https://tracker.ceph.com/issues/77276 [3] https://tracker.ceph.com/issues/78400 [4] https://tracker.ceph.com/issues/81915 [5] https://bugzilla.proxmox.com/show_bug.cgi?id=7784 Signed-off-by: Kefu Chai --- pveceph.adoc | 21 ++++++++++++++++----- 1 file changed, 16 insertions(+), 5 deletions(-) diff --git a/pveceph.adoc b/pveceph.adoc index 90ee35d..62405d4 100644 --- a/pveceph.adoc +++ b/pveceph.adoc @@ -773,11 +773,21 @@ FastEC (`allow_ec_optimizations`) Ceph Tentacle introduces a new erasure coding I/O path called FastEC, enabled per pool via the `allow_ec_optimizations` flag. FastEC adds partial reads and partial writes on EC pools, substantially improving small random -I/O performance -- the dominant access pattern of virtual machine disk I/O. -Upstream recommends it for RBD and CephFS backed storage +I/O performance -- the dominant access pattern of virtual machine disk I/O footnote:[Ceph Erasure Code {cephdocs-url}/rados/operations/erasure-code/]. +WARNING: FastEC is a young feature and is still receiving fixes for data +integrity and availability problems. Do not enable it on production pools, +particularly not on pools that already hold data. If you want to try it, do +so on a test cluster running the same Ceph version as production, with your +typical workloads. Besides regular I/O, test at least upgrading an existing +pool that holds data, discard and trim, resizing, scrubbing and repair, and +recovery after an OSD failure. Pools without FastEC are not affected. + +{pve} never enables FastEC on its own, it is only active if you set the flag +yourself. + FastEC has four requirements that the Ceph monitor enforces: * the pool must be erasure coded; @@ -833,14 +843,15 @@ If this reports a release older than `tentacle`, run IMPORTANT: `allow_ec_optimizations` is a one-way switch. Once enabled on a pool, the Ceph monitor refuses to clear the flag again, so rolling back requires draining and recreating the pool. Enable it only after validating -that FastEC works for your workload on a test pool. +that FastEC works for your workload on a test cluster. Enabling FastEC on a pool +++++++++++++++++++++++++ FastEC is enabled per pool via the `ceph` CLI. It can be set on both new -and existing (populated) pools. No data migration or re-encoding takes -place; only pool metadata changes. +and existing (populated) pools, but enabling it on a populated pool is +discouraged. No data migration or re-encoding takes place; only pool metadata +changes. Existing objects keep their old on-disk layout. To enable FastEC on a pool, run the following command against the data pool (typically named `-data`): -- 2.47.3