# Prometheus 2-hour block missing the last 1 hour **URL:** \ **Category:** Prometheus server\ **Created:** [April 13, 2026, 10:22am UTC](https://discuss.prometheus.io/t/prometheus-2-hour-block-missing-the-last-1-hour/3517 "2026-04-13T10:22:59Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![garethsrt](https://avatars.discourse-cdn.com/v4/letter/g/e495f1/32.png) [@garethsrt](https://discuss.prometheus.io/u/garethsrt)\ **Post date:** [April 13, 2026, 10:23am UTC](https://discuss.prometheus.io/t/prometheus-2-hour-block-missing-the-last-1-hour/3517/1 "2026-04-13T10:23:00Z")
Hi, I’ve noticed an unexpected thing since we just recently deployed and started using Thanos with our existing Prometheus environment (my question here is more Prometheus though - not Thanos). Regarding when TSDB blocks are created by Prometheus… **They seem to miss the last hour from the WAL**. We don’t notice this when using the Prometheus datasource (I guess it’s combining blocks and WAL data) but with the Thanos datasource it is noticeable. Incidentally, we have the recommended `storage.tsdb.min-block-duration=2h` and `storage.tsdb.max-block-duration=2h` in our Prometheus config since deploying Thanos. But what I see in the Prometheus file system (this is just after 10AM BST this morning): - A block has been created at 10 AM BST - showing at 09:00 UTC in the file system - all OK so far - But if I look at the `meta.json` file, it shows this: - “minTime”: 1776060000042, - “maxTime”: 1776067200000, - Which is from 6 AM GMT/UTC (7 AM BST) to 8 AM GMT/UTC (9 AM BST) - And if I look in the `wal` folder just after the block is created, I see a further 1 hour of files (covering, I guess, from 9 AM BST up to current 10 AM BST) So what I’m curious about is why, when a block is created at 10AM BST would it only contain the 2 hours from 7AM BST to 9AM BST and leave an hour in the `wal` folder? Our Prometheus version is 2.53.0, which admittedly is a little behind the latest. I’m not sure if this might be a daylight-saving issue. I only noticed it since we changed from GMT to BST in the UK. Has anyone experienced this before? Or does this look like a common thing with TSDB blocks being written? Thanks, G ---
**Author:** ![garethsrt](https://avatars.discourse-cdn.com/v4/letter/g/e495f1/32.png) [@garethsrt](https://discuss.prometheus.io/u/garethsrt)\ **Post date:** [April 13, 2026, 10:45am UTC](https://discuss.prometheus.io/t/prometheus-2-hour-block-missing-the-last-1-hour/3517/2 "2026-04-13T10:45:22Z")
Some further info… Here’s what these two datasources look like right now (11:42 BST) in Grafana: ![image](https://europe1.discourse-cdn.com/flex017/uploads/prometheus/original/2X/2/2455cba3177ce26cc5c4723912ba3f7f010f3448.png) ---
**Author:** ![garethsrt](https://avatars.discourse-cdn.com/v4/letter/g/e495f1/32.png) [@garethsrt](https://discuss.prometheus.io/u/garethsrt)\ **Post date:** [April 13, 2026, 10:46am UTC](https://discuss.prometheus.io/t/prometheus-2-hour-block-missing-the-last-1-hour/3517/3 "2026-04-13T10:46:23Z")
The `wal` folder (times in UTC) looks like this: ![image](https://europe1.discourse-cdn.com/flex017/uploads/prometheus/original/2X/9/99712fe8fa28c0047dbdeaf4d94678995e165da4.png) ---
**Author:** ![garethsrt](https://avatars.discourse-cdn.com/v4/letter/g/e495f1/32.png) [@garethsrt](https://discuss.prometheus.io/u/garethsrt)\ **Post date:** [April 13, 2026, 10:48am UTC](https://discuss.prometheus.io/t/prometheus-2-hour-block-missing-the-last-1-hour/3517/4 "2026-04-13T10:48:47Z")
Oh, I should mention - we are running Thanos Sidecar here too… but I have that temporarily deactivated in these examples (so I can properly show what the Thanos Store is returning) ---
**Author:** ![garethsrt](https://avatars.discourse-cdn.com/v4/letter/g/e495f1/32.png) [@garethsrt](https://discuss.prometheus.io/u/garethsrt)\ **Post date:** [April 13, 2026, 11:35am UTC](https://discuss.prometheus.io/t/prometheus-2-hour-block-missing-the-last-1-hour/3517/5 "2026-04-13T11:35:42Z")
Just to follow on the from the posts above, it is now 12:26 BST and I see this in Grafana (after a new Prometheus TSDB block creation): ![image](https://europe1.discourse-cdn.com/flex017/uploads/prometheus/original/2X/9/91842f872c0bedd5efea54c8f9fd485da53cbc47.png) (Incidentally, Thanos Sidecar also did it’s upload thing, no issues there.) I see a new block folder in the Prometheus TSDB file system (timed 11:00 UTC). And the `wal` folder now shows: ```auto Apr 13 10:03 00048231 Apr 13 10:13 00048232 Apr 13 10:22 00048233 Apr 13 10:31 00048234 Apr 13 10:40 00048235 Apr 13 10:49 00048236 Apr 13 10:58 00048237 Apr 13 11:00 00048238 Apr 13 11:09 00048239 Apr 13 11:18 00048240 Apr 13 11:25 00048241 Apr 13 11:00 checkpoint.00048230/ ``` So what I’m curious about is **why the Prometheus block creation at 11:00 UTC didn’t include the 7 files between 10:00 UTC and 11:00 UTC?** This, I think, is why I get the 1-hour “gap” when I view this via the Thanos datasource. Or, excluding the Sidecar metrics as above, Thanos metrics that are 3 hours old just before a block upload, then 1 hour old after an upload. Anyone have any ideas on that? 🤔 Thanks ---
**Author:** ![garethsrt](https://avatars.discourse-cdn.com/v4/letter/g/e495f1/32.png) [@garethsrt](https://discuss.prometheus.io/u/garethsrt)\ **Post date:** [April 14, 2026, 9:27am UTC](https://discuss.prometheus.io/t/prometheus-2-hour-block-missing-the-last-1-hour/3517/6 "2026-04-14T09:27:03Z")
Today I’m looking at `prometheus_tsdb_head_min_time_seconds` and `prometheus_tsdb_head_max_time_seconds` metrics and I see this: ![image](https://europe1.discourse-cdn.com/flex017/uploads/prometheus/original/2X/e/e6eccbb0966ef1f7787cfee77bcdde7febbc72be.png) Each two hours a block is created (all looks OK in logs - no errors or warnings) but it never flushes the entire head / wal - it always leaves 1 hour. 3 hours just before the block is created - 2 hours written - head is then 1 hour. Is this normal behaviour? ---
**Author:** ![garethsrt](https://avatars.discourse-cdn.com/v4/letter/g/e495f1/32.png) [@garethsrt](https://discuss.prometheus.io/u/garethsrt)\ **Post date:** [April 28, 2026, 11:48am UTC](https://discuss.prometheus.io/t/prometheus-2-hour-block-missing-the-last-1-hour/3517/7 "2026-04-28T11:48:18Z")
> _Is this normal behaviour?_ I’m proceeding on the assertion that this _is_ normal behaviour. I believe my initial assumption that the creation of blocks every 2 hours would include all metrics up to current time isn’t right - and that, actually, the last hour remains (probably still active, being written to, etc.). I’ve not found anything yet to confirm this absolutely, but all our Prometheus instances show the same behaviour and none is showing any errors or warnings… 🤞