diff --git a/piker/data/history.py b/piker/data/history.py
index d6f5fb5f..99fd425d 100644
--- a/piker/data/history.py
+++ b/piker/data/history.py
@@ -49,7 +49,6 @@ from ._util import (
 from ._sharedmem import (
     maybe_open_shm_array,
     ShmArray,
-    _secs_in_day,
 )
 from ._source import def_iohlcv_fields
 from ._sampling import (
@@ -65,6 +64,26 @@ if TYPE_CHECKING:
     from .feed import _FeedsBus
 
 
+# `ShmArray` buffer sizing configuration:
+_mins_in_day = int(60 * 24)
+# how much is probably dependent on lifestyle
+# but we reco a buncha times (but only on a
+# run-every-other-day kinda week).
+_secs_in_day = int(60 * _mins_in_day)
+_days_in_week: int = 7
+
+_days_worth: int = 3
+_default_hist_size: int = 6 * 365 * _mins_in_day
+_hist_buffer_start = int(
+    _default_hist_size - round(7 * _mins_in_day)
+)
+
+_default_rt_size: int = _days_worth * _secs_in_day
+# NOTE: start the append index in rt buffer such that 1 day's worth
+# can be appenened before overrun.
+_rt_buffer_start = int((_days_worth - 1) * _secs_in_day)
+
+
 def diff_history(
     array: np.ndarray,
     append_until_dt: datetime | None = None,
@@ -812,6 +831,9 @@ async def manage_history(
     # (maybe) allocate shm array for this broker/symbol which will
     # be used for fast near-term history capture and processing.
     hist_shm, opened = maybe_open_shm_array(
+        size=_default_hist_size,
+        append_start_index=_hist_buffer_start,
+
         key=f'piker.{service}[{uuid[:16]}].{fqme}.hist',
 
         # use any broker defined ohlc dtype:
@@ -829,6 +851,8 @@ async def manage_history(
         )
 
     rt_shm, opened = maybe_open_shm_array(
+        size=_default_rt_size,
+        append_start_index=_rt_buffer_start,
         key=f'piker.{service}[{uuid[:16]}].{fqme}.rt',
 
         # use any broker defined ohlc dtype:
@@ -836,7 +860,6 @@ async def manage_history(
 
         # we expect the sub-actor to write
         readonly=False,
-        size=3*_secs_in_day,
     )
 
     # (for now) set the rt (hft) shm array with space to prepend
diff --git a/piker/fsp/_api.py b/piker/fsp/_api.py
index 11d1e7dc..92f8f271 100644
--- a/piker/fsp/_api.py
+++ b/piker/fsp/_api.py
@@ -177,6 +177,7 @@ def fsp(
 def maybe_mk_fsp_shm(
     sym: str,
     target: Fsp,
+    size: int,
     readonly: bool = True,
 
 ) -> (str, ShmArray, bool):
@@ -185,7 +186,8 @@ def maybe_mk_fsp_shm(
     exists, otherwise load the shm already existing for that token.
 
     '''
-    assert isinstance(sym, str), '`sym` should be file-name-friendly `str`'
+    if not isinstance(sym, str):
+        raise ValueError('`sym: str` should be file-name-friendly')
 
     # TODO: load output types from `Fsp`
     # - should `index` be a required internal field?
@@ -204,7 +206,7 @@ def maybe_mk_fsp_shm(
 
     shm, opened = maybe_open_shm_array(
         key,
-        # TODO: create entry for each time frame
+        size=size,
         dtype=fsp_dtype,
         readonly=True,
     )
diff --git a/piker/ui/_fsp.py b/piker/ui/_fsp.py
index b4aa2b10..f00b1e3d 100644
--- a/piker/ui/_fsp.py
+++ b/piker/ui/_fsp.py
@@ -377,7 +377,7 @@ class FspAdmin:
         # TODO: make this a `.src_flume` and add
         # a `dst_flume`?
         # (=> but then wouldn't this be the most basic `Viz`?)
-        self.flume = flume
+        self.flume: Flume = flume
 
     def rr_next_portal(self) -> tractor.Portal:
         name, portal = next(self._rr_next_actor)
@@ -479,9 +479,15 @@ class FspAdmin:
         fqme: str = src_mkt.get_fqme(delim_char='')
 
         # allocate an output shm array
+
+        # NOTE: rn we assume the HFT 1s period chart
+        # is always used!
+        src_shm: ShmArray = self.flume._rt_shm
+
         key, dst_shm, opened = maybe_mk_fsp_shm(
             fqme,
             target=target,
+            size=src_shm._token.size,
             readonly=True,
         )